PC World 1999 August/PCWorld_1999-08_cd.bin/doc/HOWTO/Multi-Disk-HOWTO

Text File | 1999-05-31 | 206.4 KB | 5,677 lines

  HOWTO: Multi Disk System Tuning
  Stein Gjoen, [e-mail]
  v0.22i, 27 May 1999

  This document describes how best to use multiple disks and partitions
  for a Linux system. Although some of this text is Linux specific the
  general approach outlined here can be applied to many other multi
  tasking operating systems.
  ______________________________________________________________________

  Table of Contents























































  1. Introduction

     1.1 Copyright
     1.2 Disclaimer
     1.3 News
     1.4 Credits

  2. Structure

     2.1 Logical structure
     2.2 Document structure
     2.3 Reading plan

  3. Drive technologies

     3.1 Drives
     3.2 Geometry
     3.3 Media
        3.3.1 Magnetic Drives
        3.3.2 Optical Drives
        3.3.3 Solid State Drives
     3.4 Interfaces
        3.4.1 MFM and RLL
        3.4.2 ESDI
        3.4.3 IDE and ATA
        3.4.4 EIDE, Fast-ATA and ATA-2
        3.4.5 Ultra-ATA
        3.4.6 ATAPI
        3.4.7 SCSI
     3.5 Cabling
     3.6 Host Adapters
     3.7 Multi Channel Systems
     3.8 Multi Board Systems
     3.9 Speed Comparison
        3.9.1 Controllers
        3.9.2 Bus Types
     3.10 Benchmarking
     3.11 Comparisons
     3.12 Future Development
     3.13 Recommendations

  4. File System Structure

     4.1 File System Features
        4.1.1 Swap
        4.1.2 Temporary Storage (
        4.1.3 Spool Areas (
        4.1.4 Home Directories (
        4.1.5 Main Binaries (
        4.1.6 Libraries (
        4.1.7 Boot
        4.1.8 Root
        4.1.9 DOS etc.
     4.2 Explanation of Terms
        4.2.1 Speed
        4.2.2 Reliability
        4.2.3 Files

  5. File Systems

     5.1 General Purpose File Systems
        5.1.1 (TT
        5.1.2 (TT
        5.1.3 (TT
        5.1.4 (TT
        5.1.5 (TT
        5.1.6 (TT
        5.1.7 (TT
     5.2 Microsoft File Systems
        5.2.1 (TT
        5.2.2 (TT
        5.2.3 (TT
        5.2.4 (TT
     5.3 Logging and Journaling File Systems
     5.4 Read-only File Systems
        5.4.1 (TT
        5.4.2 (TT
        5.4.3 (TT
        5.4.4 (TT
        5.4.5 Trivia
        5.4.6 (TT
     5.5 Networking File Systems
        5.5.1 (TT
        5.5.2 (TT
        5.5.3 Coda
        5.5.4 (TT
        5.5.5 GFS
     5.6 Special File Systems
        5.6.1 (TT
        5.6.2 (TT
        5.6.3 (TT
     5.7 File System Recommendations

  6. Technologies

     6.1 RAID
        6.1.1 SCSI-to-SCSI
        6.1.2 PCI-to-SCSI
        6.1.3 Software RAID
        6.1.4 RAID Levels
     6.2 Volume Management
     6.3 Linux
     6.4 Compression
     6.5 ACL
     6.6 (TT
     6.7 Translucent or Inheriting File Systems
     6.8 Physical Track Positioning
        6.8.1 Disk Speed Values
     6.9 Stacking
     6.10 Recommendations

  7. Other Operating Systems

     7.1 DOS
     7.2 Windows
     7.3 OS/2
     7.4 NT
     7.5 Sun OS
        7.5.1 Sun OS 4
        7.5.2 Sun OS 5 (aka Solaris)
        7.5.3 BeOS

  8. Clusters

  9. Mount Points

  10. Considerations and Dimensioning

     10.1 Home Systems
     10.2 Servers
        10.2.1 Home Directories
        10.2.2 Anonymous FTP
        10.2.3 WWW
        10.2.4 Mail
        10.2.5 News
        10.2.6 Others
        10.2.7 Server Recommendations
     10.3 Pitfalls

  11. Disk Layout

     11.1 Selection for Partitioning
     11.2 Mapping Partitions to Drives
     11.3 Sorting Partitions on Drives
     11.4 Optimizing
        11.4.1 Optimizing by Characteristics
        11.4.2 Optimizing by Drive Parallelising
     11.5 Compromises

  12. Implementation

     12.1 Drives and Partitions
     12.2 Partitioning
     12.3 Repartitioning
     12.4 Microsoft Partition Bug
     12.5 Multiple Devices (
     12.6 Formatting
     12.7 Mounting
     12.8 (TT
     12.9 Recommendations

  13. Maintenance

     13.1 Backup
     13.2 Defragmentation
     13.3 Deletions
     13.4 Upgrades
     13.5 Recovery

  14. Advanced Issues

     14.1 Hard Disk Tuning
     14.2 File System Tuning
     14.3 Spindle Synchronizing

  15. Further Information

     15.1 News groups
     15.2 Mailing Lists
     15.3 HOWTO
     15.4 Mini-HOWTO
     15.5 Local Resources
     15.6 Web Pages
     15.7 Search Engines

  16. Getting Help

  17. Concluding Remarks

     17.1 Coming Soon
     17.2 Request for Information
     17.3 Suggested Project Work

  18. Questions and Answers

  19. Bits and Pieces

     19.1 Swap Partition: to Use or Not to Use
     19.2 Mount Point and
     19.3 Power and Heating
     19.4 Dejanews

  20. Appendix A: Partitioning Layout Table: Mounting and Linking

  21. Appendix B: Partitioning Layout Table: Numbering and Sizing

  22. Appendix C: Partitioning Layout Table: Partition Placement

  23. Appendix D: Example: Multipurpose Server

  24. Appendix E: Example: Mounting and Linking

  25. Appendix F: Example: Numbering and Sizing

  26. Appendix G: Example: Partition Placement

  27. Appendix H: Example II

  28. Appendix I: Example III: SPARC Solaris

  29. Appendix J: Example IV: Server with 4 Drives

  30. Appendix K: Example V: Dual Drive System

  31. Appendix L: Example VI: Single Drive System



  ______________________________________________________________________

  1.  Introduction

  In commemoration of Linux kernel 2.2 release this brand new release is
  codenamed the Daniella release.

  New code names will appear as per industry standard guidelines to
  emphasize the state-of-the-art-ness of this document.


  This document was written for two reasons, mainly because I got hold
  of 3 old SCSI disks to set up my Linux system on and I was pondering
  how best to utilise the inherent possibilities of parallelizing in a
  SCSI system. Secondly I hear there is a prize for people who write
  documents...

  This is intended to be read in conjunction with the Linux Filesystem
  Structure Standard (FSSTND). It does not in any way replace it but
  tries to suggest where physically to place directories detailed in the
  FSSTND, in terms of drives, partitions, types, RAID, file system (fs),
  physical sizes and other parameters that should be considered and
  tuned in a Linux system, ranging from single home systems to large
  servers on the Internet.


  The followup to FSSTND is called the Filesystem Hierarchy Standard
  (FHS) and covers more than Linux alone. FHS version 2.0 has been
  released but there are still a few issues to be dealt with and even
  longer before this new standard will have an impact on actual
  distributions. FHS is not yet used in any distributions but Debian has
  announced they will use it in Debian 2.1 which is their next
  distribution.

  It is also a good idea to read the Linux Installation guides
  thoroughly and if you are using a PC system, which I guess the
  majority still does, you can find much relevant and useful information
  in the FAQs for the newsgroup comp.sys.ibm.pc.hardware especially for
  storage media.

  This is also a learning experience for myself and I hope I can start
  the ball rolling with this HOWTO and that it perhaps can evolve into a
  larger more detailed and hopefully even more correct HOWTO.


  First of all we need a bit of legalese. Recent development shows it is
  quite important.


  1.1.  Copyright


  This HOWTO is copyrighted 1996 Stein Gjoen.

  Unless otherwise stated, Linux HOWTO documents are copyrighted by
  their respective authors. Linux HOWTO documents may be reproduced and
  distributed in whole or in part, in any medium physical or electronic,
  as long as this copyright notice is retained on all copies. Commercial
  redistribution is allowed and encouraged; however, the author would
  like to be notified of any such distributions.

  All translations, derivative works, or aggregate works incorporating
  any Linux HOWTO documents must be covered under this copyright notice.
  That is, you may not produce a derivative work from a HOWTO and impose
  additional restrictions on its distribution. Exceptions to these rules
  may be granted under certain conditions; please contact the Linux
  HOWTO coordinator at the address given below.

  In short, we wish to promote dissemination of this information through
  as many channels as possible. However, we do wish to retain copyright
  on the HOWTO documents, and would like to be notified of any plans to
  redistribute the HOWTOs.

  If you have questions, please contact the Linux HOWTO coordinator, at
  [e-mail] via email.


  1.2.  Disclaimer


  Use the information in this document at your own risk. I disavow any
  potential liability for the contents of this document. Use of the
  concepts, examples, and/or other content of this document is entirely
  at your own risk.

  All copyrights are owned by their owners, unless specifically noted
  otherwise.  Use of a term in this document should not be regarded as
  affecting the validity of any trademark or service mark.

  Naming of particular products or brands should not be seen as
  endorsements.

  You are strongly recommended to take a backup of your system before
  major installation and backups at regular intervals.



  1.3.  News




  This release features a major restructuring and more additions than I
  can list here especially on added file system support.

  This HOWTO now uses indexing and is based on SGMLtools version 1.0.5
  and the old version will therefore not format this document properly.

  Also quite new is a number of new translations available.


  On the development front people are concentrating their energy towards
  completing Linux 2.2 and until that is released there is not going to
  be much news on disk technology for Linux.

  Debian 2.1 is readying for release and as I use Debian for my test
  systems I will make more updates when I upgrade.


  The latest version number of this document can be gleaned from my plan
  entry if you finger
  <http://www.cs.indiana.edu/finger/nox.nyx.net/sgjoen> my Nyx account.

  Also, the latest version will be available on my web space on nyx in a
  number of formats:

  ╖  HTML <http://www.nyx.net/~sgjoen/disk.html>.

  ╖  plain ASCII text <http://www.nyx.net/~sgjoen/disk.txt>.

  ╖  compressed postscript <http://www.nyx.net/~sgjoen/disk.ps.gz>.

  ╖  SGML source <http://www.nyx.net/~sgjoen/disk.sgml>.


  A European mirror of the Multi Disk HOWTO
  <http://home.sol.no/~gjoen/stein/disk.html> just went on line.



  1.4.  Credits

  In this version I have the pleasure of acknowledging even more people
  who have contributed in one way or another:
























  ronnej (at ) ucs.orst.edu
  cm (at) kukuruz.ping.at
  armbru (at) pond.sub.org
  R.P.Blake (at) open.ac.uk
  neuffer (at) goofy.zdv.Uni-Mainz.de
  sjmudd (at) redestb.es
  nat (at) nataa.fr.eu.org
  sundbyk (at) oslo.geco-prakla.slb.com
  ggjoeen (at) online.no
  mike (at) i-Connect.Net
  roth (at) uiuc.edu
  phall (at) ilap.com
  szaka (at) mirror.cc.u-szeged.hu
  CMckeon (at) swcp.com
  kris (at) koentopp.de
  edick (at) idcomm.com
  pot (at) fly.cnuce.cnr.it
  earl (at) sbox.tu-graz.ac.at
  ebacon (at) oanet.com
  vax (at) linkdead.paranoia.com
  tschenk (at) theoffice.net
  pjfarley (at) dorsai.org
  jean (at) stat.ubc.ca
  johnf (at) whitsunday.net.au
  clasen (at) unidui.uni-duisburg.de
  eeslgw (at) ee.surrey.asc.uk
  adam (at) onshore.com
  anikolae (at) wega-fddi2.rz.uni-ulm.de
  cjaeger (at) dwave.net
  eperezte (at) c2i.net




  Special thanks go to nakano (at) apm.seikei.ac.jp for doing the
  Japanese translation <http://jf.linux.or.jp/JF/JF-ftp/other-
  formats/Disk-HOWTO/html/Disk-HOWTO.html>, general contributions as
  well as contributing an example of a computer in an academic setting,
  which is included at the end of this document.

  There are now many new translations available and special thanks go to
  the translators for the job and the input they have given:


  ╖  German Translation <http://> by chewie (at) nuernberg.netsurf.de

  ╖  Swedish Translation  <http://www.swe-doc.linux.nu> by jonah (at)
     swipnet.se

  ╖  French Translation <http://www.lri.fr/~loisel/howto/> by
     Patrick.Loiseleur (at) lri.fr




  Also DPT is acknowledged for sending me documentation on their
  controllers as well as permission to quote from the material. These
  quotes have been approved before appearing here and will be clearly
  labelled. No quotes as of yet but that is coming.

  Not many still, so please read through this document, make a
  contribution and join the elite. If I have forgotten anyone, please
  let me know.

  New in this version is an appendix with a few tables you can fill in
  for your system in order to simplify the design process.
  Any comments or suggestions can be mailed to my mail address on nyx:
  [e-mail].


  So let's cut to the chase where swap and /tmp are racing along hard
  drive...






  2.  Structure

  As this type of document is supposed to be as much for learning as a
  technical reference document I have rearranged the structure to this
  end. For the designer of a system it is more useful to have the
  information presented in terms of the goals of this exercise than from
  the point of view of the logical layer structure of the devices
  themselves. Nevertheless this document would not be complete without
  such a layer structure the computer field is so full of, so I will
  include it here as an introduction to how it works.

  It is a long time since the mini in mini-HOWTO could be defended as
  proper but I am convinced that this document is as long as it needs to
  be in order to make the right design decisions, and not longer.


  2.1.  Logical structure

  This is based on how each layer access each other, traditionally with
  the application on top and the physical layer on the bottom.  It is
  quite useful to show the interrelationship between each of the layers
  used in controlling drives.


               ___________________________________________________________
               |__     File structure          ( /usr /tmp etc)        __|
               |__     File system             (ext2fs, vfat etc)      __|
               |__     Volume management       (AFS)                   __|
               |__     RAID, concatenation     (md)                    __|
               |__     Device driver           (SCSI, IDE etc)         __|
               |__     Controller              (chip, card)            __|
               |__     Connection              (cable, network)        __|
               |__     Drive                   (magnetic, optical etc) __|
               -----------------------------------------------------------




  In the above diagram both volume management and RAID and concatenation
  are optional layers. The 3 lower layers are in hardware.  All parts
  are discussed at length later on in this document.


  2.2.  Document structure

  Most users start out with a given set of hardware and some plans on
  what they wish to achieve and how big the system should be. This is
  the point of view I will adopt in this document in presenting the
  material, starting out with hardware, continuing with design
  constraints before detailing the design strategy that I have found to
  work well.  I have used this both for my own personal computer at
  home, a multi purpose server at work and found it worked quite well.
  In addition my Japanese co-worker in this project have applied the
  same strategy on a server in an academic setting with similar success.
  Finally at the end I have detailed some configuration tables for use
  in your own design. If you have any comments regarding this or notes
  from your own design work I would like to hear from you so this
  document can be upgraded.


  2.3.  Reading plan

  Although not the biggest HOWTO it is nevertheless rather big already
  and I have been requested to make a reading plan to make it possible
  to cut down on the volume


     Expert
        (aka the elite). If you are familiar with Linux as well as disk
        drive technologies you will find most of what you need in the
        appendices. Additionally you are recommended to read the FAQ and
        the ``Bits'n'pieces'' chapter.


     Experienced
        (aka Competent). If you are familiar with computers in general
        you can go straight to the chapters on ``technologies'' and
        continue from there on.


     Newbie
        (mostly harmless). You just have to read the whole thing.
        Sorry. In addition you are also recommended to read all the
        other disk related HOWTOs.



  3.  Drive technologies

  A far more complete discussion on drive technologies for IBM PCs can
  be found at the home page of The Enhanced IDE/Fast-ATA FAQ
  <http://thef-nym.sci.kun.nl/~pieterh/storage.html> which is also
  regularly posted on Usenet News.  Here I will just present what is
  needed to get an understanding of the technology and get you started
  on your setup.


  3.1.  Drives

  This is the physical device where your data lives and although the
  operating system makes the various types seem rather similar they can
  in actual fact be very different. An understanding of how it works can
  be very useful in your design work. Floppy drives fall outside the
  scope of this document, though should there be a big demand I could
  perhaps be persuaded to add a little here.


  3.2.  Geometry

  Physically disk drives consists of one or more platters containing
  data that is read in and out using sensors mounted on movable heads
  that are fixed with respects to themselves. Data transfers therefore
  happens across all surfaces simultaneously which defines a cylinder of
  tracks. The drive is also divided into sectors containing a number of
  data fields.

  Drives are therefore often specified in terms of its geometry: the
  number of Cylinders, Heads and Sectors (CHS).


  For various reasons there is now a number of translations between

  ╖  the physical CHS of the drive itself

  ╖  the logical CHS the drive reports to the BIOS or OS

  ╖  the logical CHS used by the OS

  Basically it is a mess and a source of much confusion. For more
  information you are strongly recommended to read the Large Disk mini-
  HOWTO


  3.3.  Media

  The media technology determines important parameters such as
  read/write rates, seek times, storage size as well as if it is
  read/write or read only.


  3.3.1.  Magnetic Drives

  This is the typical read-write mass storage medium, and as everything
  else in the computer world, comes in many flavours with different
  properties. Usually this is the fastest technology and offers
  read/write capability. The platter rotates with a constant angular
  velocity (CAV) with a variable physical sector density for more
  efficient magnetic media area utilisation.  In other words, the number
  of bits per unit length is kept roughly constant by increasing the
  number of logical sectors for the outer tracks.

  Typical values for rotational speeds are 4500 and 5400 RPM, though
  7200 is also used. Very recently also 10000 RPM has entered the mass
  market.  Seek times are around 10 ms, transfer rates quite variable
  from one type to another but typically 4-40 MB/s.  With the extreme
  high performance drives you should remember that performance costs
  more electric power which is dissipated as heat, see the point on
  ``Power and Heating''.


  Note that there are several kinds of transfers going on here, and that
  these are quoted in different units. First of all there is the
  platter-to-drive cache transfer, usually quoted in Mbits/s. Typical
  values here is about 50-250 Mbits/s. The second stage is from the
  built in drive cache to the adapter, and this is typically quoted in
  MB/s, and typical quoted values here is 3-40 MB/s. Note, however, that
  this assumed data is already in the cache and hence for maximum
  readout speed from the drive the effective transfer rate will decrease
  dramatically.



  3.3.2.  Optical Drives

  Optical read/write drives exist but are slow and not so common. They
  were used in the NeXT machine but the low speed was a source for much
  of the complaints. The low speed is mainly due to the thermal nature
  of the phase change that represents the data storage. Even when using
  relatively powerful lasers to induce the phase changes the effects are
  still slower than the magnetic effect used in magnetic drives.

  Today many people use CD-ROM drives which, as the name suggests, is
  read-only. Storage is about 650 MB, transfer speeds are variable,
  depending on the drive but can exceed 1.5 MB/s. Data is stored on a
  spiraling single track so it is not useful to talk about geometry for
  this. Data density is constant so the drive uses constant linear
  velocity (CLV). Seek is also slower, about 100 ms, partially due to
  the spiraling track. Recent, high speed drives, use a mix of CLV and
  CAV in order to maximize performance. This also reduces access time
  caused by the need to reach correct rotational speed for readout.

  A new type (DVD) is on the horizon, offering up to about 18 GB on a
  single disk.


  3.3.3.  Solid State Drives

  This is a relatively recent addition to the available technology and
  has been made popular especially in portable computers as well as in
  embedded systems. Containing no movable parts they are very fast both
  in terms of access and transfer rates. The most popular type is flash
  RAM, but also other types of RAM is used. A few years ago many had
  great hopes for magnetic bubble memories but it turned out to be
  relatively expensive and is not that common.

  In general the use of RAM disks are regarded as a bad idea as it is
  normally more sensible to add more RAM to the motherboard and let the
  operating system divide the memory pool into buffers, cache, program
  and data areas. Only in very special cases, such as real time systems
  with short time margins, can RAM disks be a sensible solution.

  Flash RAM is today available in several 10's of megabytes in storage
  and one might be tempted to use it for fast, temporary storage in a
  computer. There is however a huge snag with this: flash RAM has a
  finite life time in terms of the number of times you can rewrite data,
  so putting swap, /tmp or /var/tmp on such a device will certainly
  shorten its lifetime dramatically.  Instead, using flash RAM for
  directories that are read often but rarely written to, will be a big
  performance win.

  In order to get the optimum life time out of flash RAM you will need
  to use special drivers that will use the RAM evenly and minimize the
  number of block erases.

  This example illustrates the advantages of splitting up your directory
  structure over several devices.

  Solid state drives have no real cylinder/head/sector addressing but
  for compatibility reasons this is simulated by the driver to give a
  uniform interface to the operating system.


  3.4.  Interfaces

  There is a plethora of interfaces to chose from widely ranging in
  price and performance. Most motherboards today include IDE interface
  which are part of modern chipsets.

  Many motherboards also include a SCSI interface chip made by Symbios
  (formerly NCR) and that is connected directly to the PCI bus.  Check
  what you have and what BIOS support you have with it.


  3.4.1.  MFM and RLL

  Once upon a time this was the established technology, a time when 20
  MB was awesome, which compared to todays sizes makes you think that
  dinosaurs roamed the Earth with these drives. Like the dinosaurs these
  are outdated and are slow and unreliable compared to what we have
  today. Linux does support this but you are well advised to think twice
  about what you would put on this. One might argue that an emergency
  partition with a suitable vintage of DOS might be fitting.
  3.4.2.  ESDI

  Actually, ESDI was an adaptation of the very widely used SMD interface
  used on "big" computers to the cable set used with the ST506
  interface, which was more convenient to package than the 60-pin +
  26-pin connector pair used with SMD.  The ST506 was a "dumb" interface
  which relied entirely on the controller and host computer to do
  everything from computing head/cylinder/sector locations and keeping
  track of the head location, etc. ST506 required the controller to
  extract clock from the recovered data, and control the physical
  location of detailed track features on the medium, bit by bit. It had
  about a 10-year life if you include the use of MFM, RLL, and ERLL/ARLL
  modulation schemes. ESDI, on the other hand, had intelligence, often
  using three or four separate microprocessors on a single drive, and
  high-level commands to format a track, transfer data, perform seeks,
  and so on. Clock recovery from the data stream was accomplished at the
  drive, which drove the clock line and presented its data in NRZ,
  though error correction was still the task of the controller.  ESDI
  allowed the use of variable bit density recording, or, for that
  matter, any other modulation technique, since it was locally generated
  and resolved at the drive. Though many of the techniques used in ESDI
  were later incorporated in IDE, it was the increased popularity of
  SCSI which led to the demise of ESDI in computers. ESDI had a life of
  about 10 years, though mostly in servers and otherwise "big" systems
  rather than PC's.



  3.4.3.  IDE and ATA

  Progress made the drive electronics migrate from the ISA slot card
  over to the drive itself and Integrated Drive Electronics was borne.
  It was simple, cheap and reasonably fast so the BIOS designers
  provided the kind of snag that the computer industry is so full of. A
  combination of an IDE limitation of 16 heads together with the BIOS
  limitation of 1024 cylinders gave us the infamous 504 MB limit.
  Following the computer industry traditions again, the snag was patched
  with a kludge and we got all sorts of translation schemes and BIOS
  bodges. This means that you need to read the installation
  documentation very carefully and check up on what BIOS you have and
  what date it has as the BIOS has to tell Linux what size drive you
  have. Fortunately with Linux you can also tell the kernel directly
  what size drive you have with the drive parameters, check the
  documentation for LILO and Loadlin, thoroughly. Note also that IDE is
  equivalent to ATA, AT Attachment.  IDE uses CPU-intensive Programmed
  Input/Output (PIO) to transfer data to and from the drives and has no
  capability for the more efficient Direct Memory Access (DMA)
  technology. Highest transfer rate is 8.3 MB/s.


  3.4.4.  EIDE, Fast-ATA and ATA-2

  These 3 terms are roughly equivalent, fast-ATA is ATA-2 but EIDE
  additionally includes ATAPI. ATA-2 is what most use these days which
  is faster and with DMA. Highest transfer rate is increased to 16.6
  MB/s.



  3.4.5.  Ultra-ATA

  A new, faster DMA mode that is approximately twice the speed of EIDE
  PIO-Mode 4 (33 MB/s). Disks with and without Ultra-ATA can be mixed on
  the same cable without speed penalty for the faster adapters. The
  Ultra-ATA interface is electrically identical with the normal Fast-ATA
  interface, including the maximum cable length.
  3.4.6.  ATAPI

  The ATA Packet Interface was designed to support CD-ROM drives using
  the IDE port and like IDE it is cheap and simple.


  3.4.7.  SCSI

  The Small Computer System Interface is a multi purpose interface that
  can be used to connect to everything from drives, disk arrays,
  printers, scanners and more. The name is a bit of a misnomer as it has
  traditionally been used by the higher end of the market as well as in
  work stations since it is well suited for multi tasking environments.

  The standard interface is 8 bits wide and can address 8 devices.
  There is a wide version with 16 bit that is twice as fast on the same
  clock and can address 16 devices. The host adapter always counts as a
  device and is usually number 7.  It is also possible to have 32 bit
  wide busses but this usually requires a double set of cables to carry
  all the lines.

  The old standard was 5 MB/s and the newer fast-SCSI increased this to
  10 MB/s. Recently ultra-SCSI, also known as Fast-20, arrived with 20
  MB/s transfer rates for an 8 bit wide bus.  New low voltage
  differential (LVD) signalling allows these high speeds as well as much
  longer cabling than before.

  Even more recently an even faster standard has been proposed: SCSI
  160/m which is capable of a monstrous 160 MB/s over a 16 bit wide bus.
  Support is scarce yet but for a few 10000 RPM drives that can transfer
  40 MB/s sustained.  Putting 6 such drives on a RAID will keep such a
  bus saturated and also saturate most PCI busses. Obviously this is
  only for the very highest end servers per today.

  The higher performance comes at a cost that is usually higher than for
  (E)IDE. The importance of correct termination and good quality cables
  cannot be overemphasized. SCSI drives also often tend to be of a
  higher quality than IDE drives. Also adding SCSI devices tend to be
  easier than adding more IDE drives: Often it is only a matter of
  plugging or unplugging the device; some people do this without
  powering down the system. This feature is most convenient when you
  have multiple systems and you can just take the devices from one
  system to the other should one of them fail for some reason.

  There is a number of useful documents you should read if you use SCSI,
  the SCSI HOWTO as well as the SCSI FAQ posted on Usenet News.

  SCSI also has the advantage you can connect it easily to tape drives
  for backing up your data, as well as some printers and scanners. It is
  even possible to use it as a very fast network between computers while
  simultaneously share SCSI devices on the same bus. Work is under way
  but due to problems with ensuring cache coherency between the
  different computers connected, this is a non trivial task.

  SCSI numbers are also used for arbitration. If several drives request
  service, the drive with the lowest number is given priority.



  3.5.  Cabling


  I do not intend to make too many comments on hardware but I feel I
  should make a little note on cabling. This might seem like a
  remarkably low technological piece of equipment, yet sadly it is the
  source of many frustrating problems. At todays high speeds one should
  think of the cable more of a an RF device with its inherent demands on
  impedance matching. If you do not take your precautions you will get a
  much reduced reliability or total failure. Some SCSI host adapters are
  more sensitive to this than others.

  Shielded cables are of course better than unshielded but the price is
  much higher. With a little care you can get good performance from a
  cheap unshielded cable.


  ╖  For Fast-ATA and Ultra-ATA, the maximum cable length is specified
     as 45cm (18"). The data lines of both IDE channels are connected on
     many boards, though, so they count as one cable. In any case EIDE
     cables should be as short as possible. If there are mysterious
     crashes or spontaneous changes of data, it is well worth
     investigating your cabling.  Try a lower PIO mode or disconnect the
     second channel and see if the problem still occurs.

  ╖  Use as short cable as possible, but do not forget the 30 cm minimum
     separation for ultra SCSI and 60 cm separation for differential
     SCSI.

  ╖  Avoid long stubs between the cable and the drive, connect the plug
     on the cable directly to the drive without an extension.

  ╖  SCSI Cabling limitations:



       Bus Speed (MHz)         |    Max Length (m)
       --------------------------------------------------
        5                      |        6
       10  (fast)              |        3
       20  (fast-20 / ultra)   |        3 (max 4 devices), 1.5 (max 8 devices)
       xx  (differential)      |       25 (max 16 devices
       --------------------------------------------------





  ╖  Use correct termination for SCSI devices and at the correct
     positions: both ends of the SCSI chain. Remember the host adapter
     itself may have on board termination.

  ╖  Do not mix shielded or unshielded cabling, do not wrap cables
     around metal, try to avoid proximity to metal parts along parts of
     the cabling. Any such discontinuities can cause impedance
     mismatching which in turn can cause reflection of signals which
     increases noise on the cable.  This problems gets even more severe
     in the case of multi channel controllers.  Recently someone
     suggested wrapping bubble plastic around the cables in order to
     avoid too close proximity to metal, a real problem inside crowded
     cabinets.

  More information on SCSI cabling and termination can be found at other
  <http://resource.simplenet.com/files/68_50_n.htm> web pages around the
  net.



  3.6.  Host Adapters


  This is the other end of the interface from the drive, the part that
  is connected to a computer bus. The speed of the computer bus and that
  of the drives should be roughly similar, otherwise you have a
  bottleneck in your system. Connecting a RAID 0 disk-farm to a ISA card
  is pointless. These days most computers come with 32 bit PCI bus
  capable of 132 MB/s transfers which should not represent a bottleneck
  for most people in the near future.

  As the drive electronic migrated to the drives the remaining part that
  became the (E)IDE interface is so small it can easily fit into the PCI
  chip set. The SCSI host adapter is more complex and often includes a
  small CPU of its own and is therefore more expensive and not
  integrated into the PCI chip sets available today. Technological
  evolution might change this.

  Some host adapters come with separate caching and intelligence but as
  this is basically second guessing the operating system the gains are
  heavily dependent on which operating system is used. Some of the more
  primitive ones, that shall remain nameless, experience great gains.
  Linux, on the other hand, have so much smarts of its own that the
  gains are much smaller.

  Mike Neuffer, who did the drivers for the DPT controllers, states that
  the DPT controllers are intelligent enough that given enough cache
  memory it will give you a big push in performance and suggests that
  people who have experienced little gains with smart controllers just
  have not used a sufficiently intelligent caching controller.


  3.7.  Multi Channel Systems

  In order to increase throughput it is necessary to identify the most
  significant bottlenecks and then eliminate them. In some systems, in
  particular where there are a great number of drives connected, it is
  advantageous to use several controllers working in parallel, both for
  SCSI host adapters as well as IDE controllers which usually have 2
  channels built in. Linux supports this.

  Some RAID controllers feature 2 or 3 channels and it pays to spread
  the disk load across all channels. In other words, if you have two
  SCSI drives you want to RAID and a two channel controller, you should
  put each drive on separate channels.


  3.8.  Multi Board Systems

  In addition to having both a SCSI and an IDE in the same machine it is
  also possible to have more than one SCSI controller. Check the SCSI-
  HOWTO on what controllers you can combine. Also you will most likely
  have to tell the kernel it should probe for more than just a single
  SCSI or a single IDE controller. This is done using kernel parameters
  when booting, for instance using LILO.  Check the HOWTOs for SCSI and
  LILO for how to do this.

  Multi board systems can offer significant speed gains if you configure
  your disks right, especially for RAID0. Make sure you interleave the
  controllers as well as the drives, so that you add drives to the md
  RAID device in the right order.  If controller 1 is connected to
  drives sda and sdc while controller 2 is connected to drives sdb and
  sdd you will gain more paralellicity by adding in the order of sda -
  sdc - sdb - sdd rather than sda - sdb - sdc - sdd because a read or
  write over more then one cluster will be more likely to span two
  controllers.

  The same methods can also be applied to IDE. Most motherboards come
  with typically 4 IDE ports:


  ╖  hda primary controller

  ╖  hdb primary slave

  ╖  hdc secondary controller

  ╖  hdd secondary slave

     where the two primaries share one flat cable and the secondaries
     share another cable. Modern chipsets keep these independent.
     Therefore it is best to RAID in the order hda - hdc - hdb - hdd as
     this will most likely parallelise both channels.


  3.9.  Speed Comparison

  The following tables are given just to indicate what speeds are
  possible but remember that these are the theoretical maximum speeds.
  All transfer rates are in MB per second and bus widths are measured in
  bits.



  3.9.1.  Controllers



       IDE             :        8.3 - 16.7
       Ultra-ATA       :       33

       SCSI            :
                               Bus width (bits)

       Bus Speed (MHz)         |        8      16      32
       --------------------------------------------------
        5                      |        5      10      20
       10  (fast)              |       10      20      40
       20  (fast-20 / ultra)   |       20      40      80
       40  (fast-40 / ultra-2) |       40      80      --
       --------------------------------------------------






  3.9.2.  Bus Types




       ISA             :        8-12
       EISA            :       33
       VESA            :       40    (Sometimes tuned to 50)

       PCI
                               Bus width (bits)

       Bus Speed (MHz)         |       32      64
       --------------------------------------------------
       33                      |       132     264
       66                      |       264     528
       --------------------------------------------------



  3.10.  Benchmarking

  This is a very, very difficult topic and I will only make a few
  cautious comments about this minefield. First of all, it is more
  difficult to make comparable benchmarks that have any actual meaning.
  This, however, does not stop people from trying...

  Instead one can use benchmarking to diagnose your own system, to check
  it is going as fast as it should, that is, not slowing down.  Also you
  would expect a significant increase when switching from a simple file
  system to RAID, so a lack of performance gain will tell you something
  is wrong.

  When you try to benchmark you should not hack up your own, instead
  look up iozone and bonnie and read the documentation very carefully.
  In particular make sure your buffer size is bigger than your RAM size,
  otherwise you test your RAM rather than your disks which will give you
  unrealistically high performance.

  A very simple benchmark can be obtained using hdparm -tT which can be
  used both on IDE and SCSI drives.

  For more information on benchmarking and software for a number of
  platforms, check out ACNC <http://www.acnc.com/benchmarks.html>
  benchmark page.



  3.11.  Comparisons

  SCSI offers more performance than EIDE but at a price.  Termination is
  more complex but expansion not too difficult.  Having more than 4 (or
  in some cases 2) IDE drives can be complicated, with wide SCSI you can
  have up to 15 per adapter.  Some SCSI host adapters have several
  channels thereby multiplying the number of possible drives even
  further.

  For SCSI you have to dedicate one IRQ per host adapter which can
  control up to 15 drives. With EIDE you need one IRQ for each drive
  which can cause conflict.

  RLL and MFM is in general too old, slow and unreliable to be of much
  use.



  3.12.  Future Development


  SCSI-3 is under way and will hopefully be released soon. Faster
  devices are already being announced, recently an 80 MB/s and then a
  160 MB/s monster specification has been proposed.  These are based
  around the Ultra-2 standard (which used a 40 MHz clock) combined with
  a 16 bit cable.

  Some manufacturers already announce SCSI-3 devices but this is
  currently rather premature as the standard is not yet firm. As the
  transfer speeds increase the saturation point of the PCI bus is
  getting closer. Currently the 64 bit version has a limit of 264 MB/s.
  The PCI transfer rate will in the future be increased from the current
  33 MHz to 66 MHz, thereby increasing the limit to 528 MB/s.

  Another trend is for larger and larger drives. I hear it is possible
  to get 55 GB on a single drive though this is rather expensive.
  Currently the optimum storage for your money is about 6.4 GB but also
  this is continuously increasing. The introduction of DVD will in the
  near future have a big impact, with nearly 20 GB on a single disk you
  can have a complete copy of even major FTP sites from around the
  world. The only thing we can be reasonably sure about the future is
  that even if it won't get any better, it will definitely be bigger.

  Addendum: soon after I first wrote this I read that the maximum useful
  speed for a CD-ROM was 20x as mechanical stability would be too great
  a problem at these speeds. About one month after that again the first
  commercial 24x CD-ROMs were available... Currently you can get 40x and
  no doubt higher speeds are in the pipeline.


  3.13.  Recommendations

  My personal view is that EIDE or Ultra ATA is the best way to start
  out on your system, especially if you intend to use DOS as well on
  your machine.  If you plan to expand your system over many years or
  use it as a server I would strongly recommend you get SCSI drives.
  Currently wide SCSI is a little more expensive. You are generally more
  likely to get more for your money with standard width SCSI. There is
  also differential versions of the SCSI bus which increases maximum
  length of the cable. The price increase is even more substantial and
  cannot therefore be recommended for normal users.

  In addition to disk drives you can also connect some types of scanners
  and printers and even networks to a SCSI bus.

  Also keep in mind that as you expand your system you will draw ever
  more power, so make sure your power supply is rated for the job and
  that you have sufficient cooling. Many SCSI drives offer the option of
  sequential spin-up which is a good idea for large systems.  See also
  ``Power and Heating''.






  4.  File System Structure

  Linux has been multi tasking from the very beginning where a number of
  programs interact and run continuously. It is therefore important to
  keep a file structure that everyone can agree on so that the system
  finds data where it expects to. Historically there has been so many
  different standards that it was confusing and compatibility was
  maintained using symbolic links which confused the issue even further
  and the structure ended looking like a maze.

  In the case of Linux a standard was fortunately agreed on early on
  called the File Systems Standard (FSSTND) which today is used by all
  main Linux distributions.

  Later it was decided to make a successor that should also support
  operating systems other than just Linux, called the Filesystem
  Hierarchy Standard (FHS) at version 2.1 currently.  This standard is
  is under continuous development and will soon be adopted by Linux
  distributions.

  I recommend not trying to roll your own structure as a lot of thought
  has gone into the standards and many software packages comply with the
  standards. Instead you can read more about this at the FHS home page
  <http://www.pathname.com/fhs>.

  This HOWTO endeavours to comply with FSSTND and will follow FHS when
  distributions become available.

  4.1.  File System Features

  The various parts of FSSTND have different requirements regarding
  speed, reliability and size, for instance losing root is a pain but
  can easily be recovered. Losing /var/spool/mail is a rather different
  issue. Here is a quick summary of some essential parts and their
  properties and requirements. Note that this is just a guide, there can
  be binaries in etc and lib directories, libraries in bin directories
  and so on.



  4.1.1.  Swap


     Speed
        Maximum! Though if you rely too much on swap you should consider
        buying some more RAM. Note, however, that on many PC
        motherboards the cache will not work on RAM above 128 MB.


     Size
        Similar as for RAM. Quick and dirty algorithm: just as for tea:
        16 MB for the machine and 2 MB for each user. Smallest kernel
        run in 1 MB but is tight, use 4 MB for general work and light
        applications, 8 MB for X11 or GCC or 16 MB to be comfortable.
        (The author is known to brew a rather powerful cuppa tea...)

        Some suggest that swap space should be 1-2 times the size of the
        RAM, pointing out that the locality of the programs determines
        how effective your added swap space is. Note that using the same
        algorithm as for 4BSD is slightly incorrect as Linux does not
        allocate space for pages in core.

        A more thorough approach is to consider swap space plus RAM as
        your total working set, so if you know how much space you will
        need at most, you subtract the physical RAM you have and that is
        the swap space you will need.

        There is also another reason to be generous when dimensioning
        your swap space: memory leaks. Ill behaving programs that do not
        free the memory they allocate for themselves are said to have a
        memory leak.  This allocation remains even after the offending
        program has stopped so this is a source of memory consumption.
        Once all physical RAM and swap space are exhausted the only
        solution is to reboot and start over.  Thankfully such programs
        are not too common but should you come across one you will find
        that extra swap space will buy you extra time between reboots.

        Also remember to take into account the type of programs you use.
        Some programs that have large working sets, such as finite
        element modeling (FEM) have huge data structures loaded in RAM
        rather than working explicitly on disk files. Data and computing
        intensive programs like this will cause excessive swapping if
        you have less RAM than the requirements.

        Other types of programs can lock their pages into RAM. This can
        be for security reasons, preventing copies of data reaching a
        swap device or for performance reasons such as in a real time
        module. Either way, locking pages reduces the remaining amount
        of swappable memory and can cause the system to swap earlier
        then otherwise expected.

        In man 8 mkswap it is explained that each swap partition can be
        a maximum of just under 128 MB in size for 32-bit machines and
        just under 256 MB for 64-bit machines.
     Reliability
        Medium. When it fails you know it pretty quickly and failure
        will cost you some lost work. You save often, don't you?


     Note 1
        Linux offers the possibility of interleaved swapping across
        multiple devices, a feature that can gain you much. Check out
        "man 8 swapon" for more details. However, software raiding swap
        across multiple devices adds more overheads than you gain.

        Thus the /etc/fstab file might look like this:


          /dev/sda1       swap            swap    pri=1           0       0
          /dev/sdc1       swap            swap    pri=1           0       0




     Remember that the fstab file is very sensitive to the formatting
     used, read the man page carefully and do not just cut and paste the
     lines above.


     Note 2
        Some people use a RAM disk for swapping or some other file
        systems. However, unless you have some very unusual requirements
        or setups you are unlikely to gain much from this as this cuts
        into the memory available for caching and buffering.


     Note 2b
        There is once exception: on a number of badly designed
        motherboards the on board cache memory is not able to cache all
        the RAM that can be addressed. Many older motherboards could
        accept 128 MB RAM but only cache the lower 64 MB. In such cases
        it would improve the performance if you used the upper
        (uncached) 64 MB RAM for RAMdisk based swap or other temporary
        storage.




  4.1.2.  Temporary Storage ( /tmp  and /var/tmp )


     Speed
        Very high. On a separate disk/partition this will reduce
        fragmentation generally, though ext2fs handles fragmentation
        rather well.


     Size
        Hard to tell, small systems are easy to run with just a few MB
        but these are notorious hiding places for stashing files away
        from prying eyes and quota enforcement and can grow without
        control on larger machines. Suggested: small home machine: 8 MB,
        large home machine: 32 MB, small server: 128 MB, and large
        machines up to 500 MB (The machine used by the author at work
        has 1100 users and a 300 MB /tmp directory). Keep an eye on
        these directories, not only for hidden files but also for old
        files. Also be prepared that these partitions might be the first
        reason you might have to resize your partitions.


     Reliability
        Low. Often programs will warn or fail gracefully when these
        areas fail or are filled up. Random file errors will of course
        be more serious, no matter what file area this is.


     Files
        Mostly short files but there can be a huge number of them.
        Normally programs delete their old tmp files but if somehow an
        interruption occurs they could survive. Many distributions have
        a policy regarding cleaning out tmp files at boot time, you
        might want to check out what your setup is.


     Note1
        In FSSTND there is a note about putting /tmp on RAM disk. This,
        however, is not recommended for the same reasons as stated for
        swap. Also, as noted earlier, do not use flash RAM drives for
        these directories. One should also keep in mind that some
        systems are set to automatically clean tmp areas on rebooting.


     Note2
        Older systems had a /usr/tmp but this is no longer recommended
        and for historical reasons a symbolic link now makes it point to
        one of the other tmp areas.



  (* That was 50 lines, I am home and dry! *)


  4.1.3.  Spool Areas ( /var/spool/news  and /var/spool/mail )


     Speed
        High, especially on large news servers. News transfer and
        expiring are disk intensive and will benefit from fast drives.
        Print spools: low. Consider RAID0 for news.


     Size
        For news/mail servers: whatever you can afford. For single user
        systems a few MB will be sufficient if you read continuously.
        Joining a list server and taking a holiday is, on the other
        hand, not a good idea.  (Again the machine I use at work has 100
        MB reserved for the entire /var/spool)


     Reliability
        Mail: very high, news: medium, print spool: low. If your mail is
        very important (isn't it always?) consider RAID for reliability.


     Files
        Usually a huge number of files that are around a few KB in size.
        Files in the print spool can on the other hand be few but quite
        sizable.


     Note
        Some of the news documentation suggests putting all the
        .overview files on a drive separate from the news files, check
        out all news FAQs for more information.  Typical size is about
        3-10 percent of total news spool size.

  4.1.4.  Home Directories ( /home )


     Speed
        Medium. Although many programs use /tmp for temporary storage,
        others such as some news readers frequently update files in the
        home directory which can be noticeable on large multiuser
        systems. For small systems this is not a critical issue.


     Size
        Tricky! On some systems people pay for storage so this is
        usually then a question of finance. Large systems such as
        nyx.net <http://www.nyx.net/> (which is a free Internet service
        with mail, news and WWW services) run successfully with a
        suggested limit of 100 KB per user and 300 KB as enforced
        maximum. Commercial ISPs offer typically about 5 MB in their
        standard subscription packages.

        If however you are writing books or are doing design work the
        requirements balloon quickly.


     Reliability
        Variable. Losing /home on a single user machine is annoying but
        when 2000 users call you to tell you their home directories are
        gone it is more than just annoying. For some their livelihood
        relies on what is here. You do regular backups of course?


     Files
        Equally tricky. The minimum setup for a single user tends to be
        a dozen files, 0.5 - 5 KB in size. Project related files can be
        huge though.


     Note1
        You might consider RAID for either speed or reliability. If you
        want extremely high speed and reliability you might be looking
        at other operating system and hardware platforms anyway.  (Fault
        tolerance etc.)


     Note2
        Web browsers often use a local cache to speed up browsing and
        this cache can take up a substantial amount of space and cause
        much disk activity. There are many ways of avoiding this kind of
        performance hits, for more information see the sections on
        ``Home Directories'' and ``WWW''.


     Note3
        Users often tend to use up all available space on the /home
        partition. The Linux Quota subsystem is capable of limiting the
        number of blocks and the number of inode a single user ID can
        allocate on a per-filesystem basis. See the Linux Quota mini-
        HOWTO <http://metalab.unc.edu/LDP/mini> by Albert M.C. Tam
        bertie (at) scn.org for details on setup.




  4.1.5.  Main Binaries ( /usr/bin  and /usr/local/bin )



     Speed
        Low. Often data is bigger than the programs which are demand
        loaded anyway so this is not speed critical. Witness the
        successes of live file systems on CD ROM.


     Size
        The sky is the limit but 200 MB should give you most of what you
        want for a comprehensive system. A big system, for software
        development or a multi purpose server should perhaps reserve 500
        MB both for installation and for growth.


     Reliability
        Low. This is usually mounted under root where all the essentials
        are collected. Nevertheless losing all the binaries is a pain...


     Files
        Variable but usually of the order of 10 - 100 KB.



  4.1.6.  Libraries ( /usr/lib  and /usr/local/lib )


     Speed
        Medium. These are large chunks of data loaded often, ranging
        from object files to fonts, all susceptible to bloating. Often
        these are also loaded in their entirety and speed is of some use
        here.


     Size
        Variable. This is for instance where word processors store their
        immense font files. The few that have given me feedback on this
        report about 70 MB in their various lib directories.  A rather
        complete Debian 1.2 installation can take as much as 250 MB
        which can be taken as an realistic upper limit.  The following
        ones are some of the largest disk space consumers: GCC, Emacs,
        TeX/LaTeX, X11 and perl.


     Reliability
        Low. See point ``Main binaries''.


     Files
        Usually large with many of the order of 1 MB in size.


     Note
        For historical reasons some programs keep executables in the lib
        areas. One example is GCC which have some huge binaries in the
        /usr/lib/gcc/lib hierarchy.


  4.1.7.  Boot


     Speed
        Quite low: after all booting doesn't happen that often and
        loading the kernel is just a tiny fraction of the time it takes
        to get the system up and running.


     Size
        Quite small, a complete image with some extras fit on a single
        floppy so 5 MB should be plenty.


     Reliability
        High. See section below on Root.


     Note 1
        The most important part about the Boot partition is that on many
        systems it must reside below cylinder 1023. This is a BIOS
        limitation that Linux cannot get around.



  4.1.8.  Root


     Speed
        Quite low: only the bare minimum is here, much of which is only
        run at startup time.


     Size
        Relatively small. However it is a good idea to keep some
        essential rescue files and utilities on the root partition and
        some keep several kernel versions. Feedback suggests about 20 MB
        would be sufficient.


     Reliability
        High. A failure here will possibly cause a fair bit of grief and
        you might end up spending some time rescuing your boot
        partition. With some practice you can of course do this in an
        hour or so, but I would think if you have some practice doing
        this you are also doing something wrong.

        Naturally you do have a rescue disk? Of course this is updated
        since you did your initial installation? There are many ready
        made rescue disks as well as rescue disk creation tools you
        might find valuable.  Presumably investing some time in this
        saves you from becoming a root rescue expert.


     Note 1
        If you have plenty of drives you might consider putting a spare
        emergency boot partition on a separate physical drive. It will
        cost you a little bit of space but if your setup is huge the
        time saved, should something fail, will be well worth the extra
        space.


     Note 2
        For simplicity and also in case of emergencies it is not
        advisable to put the root partition on a RAID level 0 system.
        Also if you use RAID for your boot partition you have to
        remember to have the md option turned on for your emergency
        kernel.


     Note 3
        For simplicity it is quite common to keep Boot and Root on the
        same partition. if you do that, then in order to boot from LILO
        it is important that the essential boot files reside wholly
        within cylinder 1023. This includes the kernel as well as files
        found in /boot.



  4.1.9.  DOS etc.

  At the danger of sounding heretical I have included this little
  section about something many reading this document have strong
  feelings about.  Unfortunately many hardware items come with setup and
  maintenance tools based around those systems, so here goes.


     Speed
        Very low. The systems in question are not famed for speed so
        there is little point in using prime quality drives.
        Multitasking or multi-threading are not available so the command
        queueing facility found in SCSI drives will not be taken
        advantage of. If you have an old IDE drive it should be good
        enough. The exception is to some degree Win95 and more notably
        NT which have multi-threading support which should theoretically
        be able to take advantage of the more advanced features offered
        by SCSI devices.


     Size
        The company behind these operating systems is not famed for
        writing tight code so you have to be prepared to spend a few
        tens of MB depending on what version you install of the OS or
        Windows. With an old version of DOS or Windows you might fit it
        all in on 50 MB.


     Reliability
        Ha-ha. As the chain is no stronger than the weakest link you can
        use any old drive. Since the OS is more likely to scramble
        itself than the drive is likely to self destruct you will soon
        learn the importance of keeping backups here.

        Put another way: "Your mission, should you choose to accept it,
        is to keep this partition working. The warranty will self
        destruct in 10 seconds..."

        Recently I was asked to justify my claims here. First of all I
        am not calling DOS and Windows sorry excuses for operating
        systems. Secondly there are various legal issues to be taken
        into account. Saying there is a connection between the last two
        sentences are merely the ravings of the paranoid. Surely.
        Instead I shall offer the esteemed reader a few key words: DOS
        4.0, DOS 6.x and various drive compression tools that shall
        remain nameless.




  4.2.  Explanation of Terms

  Naturally the faster the better but often the happy installer of Linux
  has several disks of varying speed and reliability so even though this
  document describes performance as 'fast' and 'slow' it is just a rough
  guide since no finer granularity is feasible. Even so there are a few
  details that should be kept in mind:





  4.2.1.  Speed

  This is really a rather woolly mix of several terms: CPU load,
  transfer setup overhead, disk seek time and transfer rate. It is in
  the very nature of tuning that there is no fixed optimum, and in most
  cases price is the dictating factor. CPU load is only significant for
  IDE systems where the CPU does the transfer itself but is generally
  low for SCSI, see SCSI documentation for actual numbers. Disk seek
  time is also small, usually in the millisecond range. This however is
  not a problem if you use command queueing on SCSI where you then
  overlap commands keeping the bus busy all the time. News spools are a
  special case consisting of a huge number of normally small files so in
  this case seek time can become more significant.

  There are two main parameters that are of interest here:


     Seek
        is usually specified in the average time take for the read/write
        head to seek from one track to another. This parameter is
        important when dealing with a large number of small files such
        as found in spool files.  There is also the extra seek delay
        before the desired sector rotates into position under the head.
        This delay is dependent on the angular velocity of the drive
        which is why this parameter quite often is quoted for a drive.
        Common values are 4500, 5400 and 7200 RPM (rotations per
        minute). Higher RPM reduces the seek time but at a substantial
        cost.  Also drives working at 7200 RPM have been known to be
        noisy and to generate a lot of heat, a factor that should be
        kept in mind if you are building a large array or "disk farm".
        Very recently drives working at 10000 RPM has entered the market
        and here the cooling requirements are even stricter and minimum
        figures for air flow are given.


     Transfer
        is usually specified in megabytes per second.  This parameter is
        important when handling large files that have to be transferred.
        Library files, dictionaries and image files are examples of
        this. Drives featuring a high rotation speed also normally have
        fast transfers as transfer speed is proportional to angular
        velocity for the same sector density.

  It is therefore important to read the specifications for the drives
  very carefully, and note that the maximum transfer speed quite often
  is quoted for transfers out of the on board cache (burst speed) and
  not directly from the platter (sustained speed).  See also section on
  ``Power and Heating''.



  4.2.2.  Reliability

  Naturally no-one would want low reliability disks but one might be
  better off regarding old disks as unreliable. Also for RAID purposes
  (See the relevant information) it is suggested to use a mixed set of
  disks so that simultaneous disk crashes become less likely.

  So far I have had only one report of total file system failure but
  here unstable hardware seemed to be the cause of the problems.

  Disks are cheap these days yet people still underestimate the value of
  the contents of the drives. If you need higher reliability make sure
  you replace old drives and keep spares. It is not unusual that drives
  can work more or less continuous for years and years but what often
  kills a drive in the end is power cycling.
  4.2.3.  Files

  The average file size is important in order to decide the most
  suitable drive parameters. A large number of small files makes the
  average seek time important whereas for big files the transfer speed
  is more important.  The command queueing in SCSI devices is very handy
  for handling large numbers of small files, but for transfer EIDE is
  not too far behind SCSI and normally much cheaper than SCSI.




  5.  File Systems

  Over time the requirements for file systems have increased and the
  demands for large structures, large files, long file names and more
  has prompted ever more advanced file systems, the system that accesses
  and organises the data on mass storage.  Today there is a large number
  of file systems to choose from and this section will describe these in
  detail.

  The emphasis is on Linux but with more input I will be happy to add
  information for a wider audience.



  5.1.  General Purpose File Systems

  Most operating systems usually have a general purpose file system for
  every day use for most kinds of files, reflecting available features
  in the OS such as permission flags, protection and recovery.


  5.1.1.  minix

  This was the original fs for Linux, back in the days Linux was hosted
  on minix machines. It is simple but limited in features and hardly
  every used these days other than in some rescue disks as it is rather
  compact.


  5.1.2.  xiafs  and extfs

  These are also old and have fallen in disuse and are no longer
  recommended.


  5.1.3.  ext2fs

  This is the established standard for general purpose in the Linux
  world.  It is fast, efficient and mature and is under continuous
  development and features such as ACL and transparent compression are
  on the horizon.

  For more information check the ext2fs
  <http://web.mit.edu/tytso/www/linux/ext2.htm> home page.



  5.1.4.  ufs

  This is the fs used by BSD and variants thereof. It is mature but also
  developed for older types of disk drives where geometries were known.
  The fs uses a number of tricks to optimise performance but as disk
  geometries are translated in a number of ways the net effect is no
  longer so optimal.
  5.1.5.  efs

  The Extent File System (efs) is Silicon Graphics' early file system
  widely used on IRIX before version 6.0 after which xfs has taken over.
  While migration to xfs is encouraged efs is still supported and much
  used on CDs.

  There is a Linux driver available in early beta stage, available at
  Linux extent file system <http://aeschi.ch.eu.org/efs/> home page.




  5.1.6.  reiserfs

  As of July, 23th 1997 Hans Reiser reiser (at) RICOCHET.NET has put up
  the source to his tree based reiserfs
  <http://idiom.com/~beverly/reiserfs.html> on the web. While his
  filesystem has some very interesting features and is much faster than
  ext2fs, it is still very experimental and difficult to integrate with
  the standard kernel. Expect some interesting developments in the
  future - this is different from your "average log based file system
  for Linux" project, because Hans already has working code.


  5.1.7.  enh-fs

  Currently in alpha stage the Enhanced File System
  <http://www.coker.com.au/~russel/enh-fs.html> project aims to combine
  file system and volume management into a single layer.




  5.2.  Microsoft File Systems

  This company is responsible for a lot, including a number of
  filesystems that has at the very least caused confusions.



  5.2.1.  fat

  Actually there are 2 fats out there, fat12 and fat16 depending on the
  partition size used but fortunately the difference is so minor that
  the whole issue is transparent.

  On the plus side these are fast and simple and most OSes understands
  it and can both read and write this fs. And that is about it.

  The minus side is limited safety, severely limited permission flags
  and atrocious scalability. For instance with fat you cannot have
  partitions larger than 2 GB.



  5.2.2.  fat32

  After about 10 years Microsoft realised fat was about, well, 10 years
  behind the times and created this fs which scales reasonably well.

  Permission flags are still limited.  NT 4.0 cannot read this file
  system but Linux can.



  5.2.3.  vfat

  At the same time as Microsoft launched fat32 they also added support
  for long file names, known as vfat.

  Linux reads vfat and fat32 partitions by mounting with type vfat.



  5.2.4.  ntfs

  This is the native fs of Win-NT but as complete information is
  available there is limited support for other OSes.



  5.3.  Logging and Journaling File Systems

  These take a radically different approach to file updates by logging
  modifications for files in a log and later at some time checkpointing
  the logs.

  Reading is roughly as fast as traditional file systems that always
  update the files directly.  Writing is much faster as only updates are
  appended to a log.  All this is transparent to the user. It is in
  reliability and particularly in checking file system integrity that
  these file systems really shine.  Since the data before last
  checkpointing is known to be good only the log has to be checked, and
  this is much faster than for traditional file systems.

  Note that while logging filesystems keep track of changes made to both
  data and inodes, journaling filesystems keep track only of inode
  changes.

  Linux has quite a choice in such file systems but none are yet in
  production quality. Some are also on hold.


  ╖  Adam Richter from Yggdrasil posted some time ago that they have
     been working on a compressed log file based system but that this
     project is currently on hold. Nevertheless a non-working version is
     available on their FTP server. Check out the Yggdrasil ftp server
     <ftp://ftp.yggdrasil.com/private/adam> where special patched
     versions of the kernel can be found.

  ╖  Another project is the Linux log-structured Filesystem Project
     <http://collective.cpoint.net/lfs/> which sadly also is on hold.
     Nevertheless this page contains much information on the topic.

  ╖  Finally there is the dtfs -- A Log-Structured Filesystem For Linux
     <http://www.complang.tuwien.ac.at/czezatke/lfs.html> which seems to
     be going strong. Still in alpha but sufficiently complete to make
     programs run off this file system


  5.4.  Read-only File Systems

  Read-only media has not escaped the ever increasing complexities seen
  in more general file systems so again there is a large choice to chose
  from with corresponding opportunities for exciting mistakes.

  Most of these are used with the CD-ROM media but also the new DVD can
  be used and you can even use it through the loopback device on a hard
  disk file for verifying an image before burning a ROM.


  There is a read-only romfs for Linux but as that is not disk related
  nothing more will be said about it here.


  5.4.1.  High Sierra

  This was one of the earliest standards for CD-ROM formats, supposedly
  named after the hotel where the final agreement took place.

  High Sierra was so limited in features that new extensions simply had
  to appear and while there has been no end to new formats the original
  High Sierra remains the common precursor and is therefore still widely
  supported.



  5.4.2.  iso9660

  The International Standards Organisation made their extensions and
  formalised the standard into what we know as the iso9660 standard.

  The Linux iso9660 file system supports both High Sierra as well as
  Rock Ridge extensions.



  5.4.3.  Rock Ridge

  Not everyone accepts limits like short filenames and lack of
  permissions so very soon the Rock Ridge extensions appeared to rectify
  these shortcomings.



  5.4.4.  Joliet

  Microsoft, not be be outdone in the standards extension game, decided
  it should extend CD-ROM formats with some internationalisation
  features and called it Joliet.

  Linux supports this standards in kernels 2.0.34 or newer.  You need to
  enable NLS in order to use it.



  5.4.5.  Trivia

  Joliet is a city outside Chicago; best known for being the site of the
  prison where Jake was locked up in the movie "Blues Brothers." Rock
  Ridge (the UNIX extensions to ISO 9660) is named after the (fictional)
  town in the movie "Blazing Saddles."



  5.4.6.  UDF

  With the arrival of DVD with up to about 17 GB of storage capacity the
  world seemingly needed another format, this time ambitiously named
  Universal Disk Format (UDF).  This is intended to replace iso9660 and
  will be required for DVD.

  Currently this is not in the standard Linux kernel but a project is
  underway to make a UDF driver
  <http://trylinux.com/projects/udf/index/htm> for Linux. Patches and
  documentation are available.

  5.5.  Networking File Systems

  There is a large number of networking technologies available that lets
  you distribute disks throughout a local or even global networks.  This
  is somewhat peripheral to the topic of this HOWTO but as it can be
  used with local disks I will cover this briefly. It would be best if
  someone (else) took this into a separate HOWTO...


  5.5.1.  NFS

  This is one of the earliest systems that allows mounting a file space
  on one machine onto another. There are a number of problems with NFS
  ranging from performance to security but it has nevertheless become
  established.


  5.5.2.  AFS

  This is a system that allows efficient sharing of files across large
  networks. Starting out as an academic project it is now sold by
  Transarc <http://www.transarc.com> whose homepage gives you more
  details.

  Derek Atkins, of MIT, ported AFS to Linux and has also set up the
  Linux AFS mailing List ( [e-mail]) for this which is open to
  the public.  Requests to join the list should go to linux-afs-
  [e-mail] and finally bug reports should be directed to linux-
  [e-mail].

  Important: as AFS uses encryption it is restricted software and cannot
  easily be exported from the US.

  IBM who owns Transarc, has announced the availability of the latest
  version of client as well as server for Linux.

  Arla is a free AFS implementation, check the Arla homepage
  <http://www.stacken.kth.se/projekt/arla> for more information as well
  as documentation.



  5.5.3.  Coda

  Work has started on a free replacement of AFS and is called Coda
  <http://coda.cs.cmu.edu/>.



  5.5.4.  nbd

  The Network Block Device <http://atrey.karlin.mff.cuni.cz/~pavel>
  (nbd) is available in Linux kernel 2.2 and later and offers reportedly
  excellent performance. The interesting thing here is that it can be
  combined with RAID (see later).



  5.5.5.  GFS

  The Global File System <http://gfs.lcse.umn.edu/> is a new file system
  designed for storage across a wide area network.  it is currently in
  the early stages and more information will come later.



  5.6.  Special File Systems

  In addition to the general file systems there is also a number of more
  specific ones, usually to provide higher performance or other
  features, usually with a tradeoff in other respects.



  5.6.1.  tmpfs  and swapfs

  For short term fast file storage SunOS offers tmpfs which is about the
  same as the swapfs on NeXT.  This overcomes the inherent slowness in
  ufs by caching file data and keeping control information in memory.
  This means that data on such a file system will be lost when rebooting
  and is therefore mainly suitable for /tmp area but not /var/tmp which
  is where temporary data that must survive a reboot, is placed.

  SunOS offers very limited tuning for tmpfs and the number of files is
  even limited by total physical memory of the machine.

  Linux does not have an equivalent to such file system and it is felt
  by many that ext2fs is fast enough to eliminate the need.



  5.6.2.  userfs

  The user file system (userfs) allows a number of extensions to
  traditional file system use such as FTP based file system, compression
  (arcfs) and fast prototyping and many other features. The docfs is
  based on this filesystem.  Check the userfs homepage
  <http://www.goop.org/~jeremy/userfs/> for more information.



  5.6.3.  devfs

  When disks are added, removed or just fail it is likely that disk
  device names of the remaining disks will change.  For instance if sdb
  fails then the old sdc becomes sdb, the old sdc becomes sdb and so on.
  Note that in this case hda, hdb etc will remain unchanged.  Likewise
  if a new drive is added the reverse may happen.

  There is no guarantee that SCSI ID 0 becomes sda and that adding disks
  in increasing ID order will just add a new device name without
  renaming previous entries, as some SCSI drivers assign from ID 0 and
  up while others reverse the scanning order.  Likewise adding a SCSI
  host adapter can also cause renaming.

  Generally device names are assigned in the order they are found.

  The source of the problem lies in the limited number of bits available
  for major and minor numbering in the device files used to describe the
  device itself. You an see these in the /dev directory, info on the
  numbering and allocation can be found in man MAKEDEV.  Currently there
  are 2 solutions to this problem in various stages of development:

     scsidev
        works by creating a database of drives and where they belong,
        check  man scsifs for more information

     devfs
        is a more long term project aimed at getting around the whole
        business of device numbering by making the /dev directory a
        kernel file system in the same way as /procfs is.  More
        information will appear as it becomes available.
  5.7.  File System Recommendations

  There is a jungle of choices but generally it is recommended to use
  the general file system that comes with your distribution.  If you use
  ufs and have some kind of tmpfs available you should first start off
  with the general file system to get an idea of the space requirements
  and if necessary buy more RAM to support the size of tmpfs you need.
  Otherwise you will end up with mysterious crashes and lost time.

  If you use dual boot and need to transfer data between the two OSes
  one of the simplest ways is to use an appropriately sized partition
  formatted with fat as most systems can reliably read and write this.
  Remember the limit of 2 GB for fat partitions.

  For more information of file system interconnectivity you can check
  out the file system <http://www.ceid.upatras.gr/~gef/fs/> page.

  To avoid total havoc with device renaming if a drive fails check out
  the scanning order of your system and try to keep your root system on
  hda or sda and removable media such as ZIP drives at the end of the
  scanning order.





  6.  Technologies

  In order to decide how to get the most of your devices you need to
  know what technologies are available and their implications. As always
  there can be some tradeoffs with respect to speed, reliability, power,
  flexibility, ease of use and complexity.

  Many of the techniques described below can be stacked in a number of
  ways to maximise performance and reliability, though at the cost of
  added complexity.



  6.1.  RAID

  This is a method of increasing reliability, speed or both by using
  multiple disks in parallel thereby decreasing access time and
  increasing transfer speed. A checksum or mirroring system can be used
  to increase reliability.  Large servers can take advantage of such a
  setup but it might be overkill for a single user system unless you
  already have a large number of disks available. See other documents
  and FAQs for more information.

  For Linux one can set up a RAID system using either software (the md
  module in the kernel), a Linux compatible controller card (PCI-to-
  SCSI) or a SCSI-to-SCSI controller. Check the documentation for what
  controllers can be used. A hardware solution is usually faster, and
  perhaps also safer, but comes at a significant cost.



  6.1.1.  SCSI-to-SCSI

  SCSI-to-SCSI controllers are usually implemented as complete cabinets
  with drives and a controller that connects to the computer with a
  second SCSI bus. This makes the entire cabinet of drives look like a
  single large, fast SCSI drive and requires no special RAID driver. The
  disadvantage is that the SCSI bus connecting the cabinet to the
  computer becomes a bottleneck.

  A significant disadvantage for people with large disk farms is that
  there is a limit to how many SCSI entries there can be in the /dev
  directory. In these cases using SCSI-to-SCSI will conserve entries.

  Usually they are configured via the front panel or with a terminal
  connected to their on-board serial interface.


  Some manufacturers of such systems are CMD <http://www.cmd.com> and
  Syred <http://www.syred.com> whose web pages describe several systems.



  6.1.2.  PCI-to-SCSI

  PCI-to-SCSI controllers are, as the name suggests, connected to the
  high speed PCI bus and is therefore not suffering from the same
  bottleneck as the SCSI-to-SCSI controllers. These controllers require
  special drivers but you also get the means of controlling the RAID
  configuration over the network which simplifies management.

  Currently only a few families of PCI-to-SCSI host adapters are
  supported under Linux.



     DPT
        The oldest and most mature is a range of controllers from DPT
        <http://www.dpt.com> including SmartCache I/III/IV and SmartRAID
        I/III/IV controller families.  These controllers are supported
        by the EATA-DMA driver in the standard kernel. This company also
        has an informative home page <http://www.dpt.com> which also
        describes various general aspects of RAID and SCSI in addition
        to the product related information.

        More information from  the author of the DPT controller drivers
        (EATA* drivers) can be found at his pages on SCSI
        <http://www.uni-mainz.de/~neuffer/scsi> and DPT <http://www.uni-
        mainz.de/~neuffer/scsi/dpt>.

        These are not the fastest but have a good track record of proven
        reliability.

        Note that the maintenance tools for DPT controllers currently
        run under DOS/Win only so you will need a small DOS/Win
        partition for some of the software. This also means you have to
        boot the system into Windows in order to maintain your RAID
        system.



     ICP-Vortex
        A very recent addition is a range of controllers from ICP-Vortex
        <http://www.icp-vortex.com> featuring up to 5 independent
        channels and very fast hardware based on the i960 chip. The
        linux driver was written by the company itself which shows they
        support Linux.

        As ICP-Vortex supplies the maintenance software for Linux it is
        not necessary with a reboot to other operating systems for the
        setup and maintenance of your RAID system. This saves you also
        extra downtime.




     Mylex DAC-960
        This is one of the latest entries which is out in early beta.
        More information as well as drivers are available at Dandelion
        Digital's Linux DAC960 Page
        <http://www.dandelion.com/Linux/DAC960.html>.



     Compaq Smart-2 PCI Disk Array Controllers
        Another very recent entry and currently in beta release is the
        Smart-2 <http://www.insync.net~frantze/cpqarray.html> driver.




  6.1.3.  Software RAID

  A number of operating systems offer software RAID using ordinary disks
  and controllers. Cost is low and performance for raw disk IO can be
  very high.  As this can be very CPU intensive it increases the load
  noticeably so if the machine is CPU bound in performance rather then
  IO bound you might be better off with a hardware PCI-to-RAID
  controller.

  Real cost, performance and especially reliability of software vs.
  hardware RAID is a very controversial topic. Reliability on Linux
  systems have been very good so far.

  The current software RAID project on Linux is the md system (multiple
  devices) which offers much more than RAID so it is described in more
  details later.




  6.1.4.  RAID Levels

  RAID comes in many levels and flavours which I will give a brief
  overview of this here. Much has been written about it and the
  interested reader is recommended to read more about this in the RAID
  FAQ.


  ╖  RAID 0 is not redundant at all but offers the best throughput of
     all levels here. Data is striped across a number of drives so read
     and write operations take place in parallel across all drives. On
     the other hand if a single drive fail then everything is lost. Did
     I mention backups?

  ╖  RAID 1 is the most primitive method of obtaining redundancy by
     duplicating data across all drives. Naturally this is massively
     wasteful but you get one substantial advantage which is fast
     access.  The drive that access the data first wins. Transfers are
     not any faster than for a single drive, even though you might get
     some faster read transfers by using one track reading per drive.

     Also if you have only 2 drives this is the only method of achieving
     redundancy.

  ╖  RAID 2 and 4 are not so common and are not covered here.

  ╖  RAID 3 uses a number of disks (at least 2) to store data in a
     striped RAID 0 fashion. It also uses an additional redundancy disk
     to store the XOR sum of the data from the data disks. Should the
     redundancy disk fail, the system can continue to operate as if
     nothing happened. Should any single data disk fail the system can
     compute the data on this disk from the information on the
     redundancy disk and all remaining disks. Any double fault will
     bring the whole RAID set off-line.

     RAID 3 makes sense only with at least 2 data disks (3 disks
     including the redundancy disk). Theoretically there is no limit for
     the number of disks in the set, but the probability of a fault
     increases with the number of disks in the RAID set. Usually the
     upper limit is 5 to 7 disks in a single RAID set.

     Since RAID 3 stores all redundancy information on a dedicated disk
     and since this information has to be updated whenever a write to
     any data disk occurs, the overall write speed of a RAID 3 set is
     limited by the write speed of the redundancy disk. This, too, is a
     limit for the number of disks in a RAID set. The overall read speed
     of a RAID 3 set with all data disks up and running is that of a
     RAID 0 set with that number of data disks. If the set has to
     reconstruct data stored on a failed disk from redundant
     information, the performance will be severely limited: All disks in
     the set have to be read and XOR-ed to compute the missing
     information.

  ╖  RAID 5 is just like RAID 3, but the redundancy information is
     spread on all disks of the RAID set. This improves write
     performance, because load is distributed more evenly between all
     available disks.

  There are also hybrids available based on RAID 0 or 1 and one other
  level. Many combinations are possible but I have only seen a few
  referred to. These are more complex than the above mentioned RAID
  levels.

  RAID 0/1 combines striping with duplication which gives very high
  transfers combined with fast seeks as well as redundancy. The
  disadvantage is high disk consumption as well as the above mentioned
  complexity.

  RAID 1/5 combines the speed and redundancy benefits of RAID5 with the
  fast seek of RAID1. Redundancy is improved compared to RAID 0/1 but
  disk consumption is still substantial. Implementing such a system
  would involve typically more than 6 drives, perhaps even several
  controllers or SCSI channels.



  6.2.  Volume Management

  Volume management is a way of overcoming the constraints of fixed
  sized partitions and disks while still having a control of where
  various parts of file space resides. With such a system you can add
  new disks to your system and add space from this drive to parts of the
  file space where needed, as well as migrating data out from a disk
  developing faults to other drives before catastrophic failure occurs.

  The system developed by Veritas <http://www.veritas.com> has become
  the defacto standard for logical volume management.

  Volume management is for the time being an area where Linux is
  lacking.

  One is the virtual partition system project VPS
  <http://www.uiuc.edu/ph/www/roth> that will reimplement many of the
  volume management functions found in IBM's AIX system. Unfortunately
  this project is currently on hold.


  Another project is the Logical Volume Manager
  <http://linux.msede.com/lvm/> project that is similar to a project by
  HP.



  6.3.  Linux md  Kernel Patch

  The Linux Multi Disk (md) provides a number of block level features in
  various stages of development.

  RAID 0 (striping) and concatenation are very solid and in production
  quality and also RAID 4 and 5 are quite mature.

  It is also possible to stack some levels, for instance mirroring (RAID
  1) two pairs of drives, each pair set up as striped disks (RAID 0),
  which offers the speed of RAID 0 combined with the reliability of RAID
  1.

  In addition to RAID this system offers (in alpha stage) block level
  volume management and soon also translucent file space.  Since this is
  done on the block level it can be used in combination with any file
  system, even for fat using Wine.

  Think very carefully what drives you combine so you can operate all
  drives in parallel, which gives you better performance and less wear.
  Read more about this in the documentation that comes with md.

  Unfortunately the documentation is rather old an in parts misleading
  and only refers to md version 0.35 which uses old style setup.  The
  new system is very different and will soon be released as version 1.0
  but is currently undocumented. If you wish to try it out you should
  follow the linux-raid mailing list.

  Documentation is improving and a Software RAID HOWTO
  <http://ostenfeld.dk/~jakob/Software-RAID.HOWTO/> is in progress.


  Hint: if you cannot get it to work properly you have forgotten to set
  the persistent-block flag. Your best documentation is currently the
  source code.




  6.4.  Compression

  Disk compression versus file compression is a hotly debated topic
  especially regarding the added danger of file corruption. Nevertheless
  there are several options available for the adventurous
  administrators. These take on many forms, from kernel modules and
  patches to extra libraries but note that most suffer various forms of
  limitations such as being read-only. As development takes place at
  neck breaking speed the specs have undoubtedly changed by the time you
  read this. As always: check the latest updates yourself. Here only a
  few references are given.


  ╖  DouBle features file compression with some limitations.

  ╖  Zlibc adds transparent on-the-fly decompression of files as they
     load.

  ╖  there are many modules available for reading compressed files or
     partitions that are native to various other operating systems
     though currently most of these are read-only.
  ╖  dmsdos <http://bf9nt.uni-
     duisburg.de/mitarbeiter/gockel/software/dmsdos/> (currently in
     version 0.9.2.0) offer many of the compression options available
     for DOS and Windows. It is not yet complete but work is ongoing and
     new features added regularly.

  ╖  e2compr is a package that extends ext2fs with compression
     capabilities. It is still under testing and will therefore mainly
     be of interest for kernel hackers but should soon gain stability
     for wider use.  Check the e2compr homepage
     <http://netspace.net.au/~reiter/e2compr.html> for more information.
     I have reports of speed and good stability which is why it is
     mentioned here.



  6.5.  ACL

  Access Control List (ACL) offers finer control over file access on a
  user by user basis, rather than the traditional owner, group and
  others, as seen in directory listings (drwxr-xr-x). This is currently
  not available in Linux but is expected in kernel 2.3 as hooks are
  already in place in ext2fs.



  6.6.  cachefs

  This uses part of a hard disk to cache slower media such as CD-ROM.
  It is available under SunOS but not yet for Linux.



  6.7.  Translucent or Inheriting File Systems

  This is a copy-on-write system where writes go to a different system
  than the original source while making it look like an ordinary file
  space. Thus the file space inherits the original data and the
  translucent write back buffer can be private to each user.

  There is a number of applications:

  ╖  updating a live file system on CD-ROM, making it flexible, fast
     while also conserving space,

  ╖  original skeleton files for each new user, saving space since the
     original data is kept in a single space and shared out,

  ╖  parallel project development prototyping where every user can
     seemingly modify the system globally while not affecting other
     users.

  SunOS offers this feature and this is under development for Linux.
  There was an old project called the Inheriting File Systems (ifs) but
  this project has stopped.  One current project is part of the md
  system and offers block level translucence so it can be applied to any
  file system.

  Sun has an informative page <http://www.sun.ca/white-papers/tfs.html>
  on translucent file system.






  6.8.  Physical Track Positioning

  This trick used to be very important when drives were slow and small,
  and some file systems used to take the varying characteristics into
  account when placing files. Although higher overall speed, on board
  drive and controller caches and intelligence has reduced the effect of
  this.

  Nevertheless there is still a little to be gained even today.  As we
  know, "world dominance" is soon within reach but to achieve this
  "fast" we need to employ all the tricks we can use .

  To understand the strategy we need to recall this near ancient piece
  of knowledge and the properties of the various track locations.  This
  is based on the fact that transfer speeds generally increase for
  tracks further away from the spindle, as well as the fact that it is
  faster to seek to or from the central tracks than to or from the inner
  or outer tracks.

  Most drives use disks running at constant angular velocity but use
  (fairly) constant data density across all tracks. This means that you
  will get much higher transfer rates on the outer tracks than on the
  inner tracks; a characteristics which fits the requirements for large
  libraries well.

  Newer disks use a logical geometry mapping which differs from the
  actual physical mapping which is transparently mapped by the drive
  itself.  This makes the estimation of the "middle" tracks a little
  harder.

  In most cases track 0 is at the outermost track and this is the
  general assumption most people use. Still, it should be kept in mind
  that there are no guarantees this is so.



     Inner
        tracks are usually slow in transfer, and lying at one end of the
        seeking position it is also slow to seek to.

        This is more suitable to the low end directories such as DOS,
        root and print spools.


     Middle
        tracks are on average faster with respect to transfers than
        inner tracks and being in the middle also on average faster to
        seek to.

        This characteristics is ideal for the most demanding parts such
        as swap, /tmp and /var/tmp.


     Outer
        tracks have on average even faster transfer characteristics but
        like the inner tracks are at the end of the seek so
        statistically it is equally slow to seek to as the inner tracks.

        Large files such as libraries would benefit from a place here.


  Hence seek time reduction can be achieved by positioning frequently
  accessed tracks in the middle so that the average seek distance and
  therefore the seek time is short. This can be done either by using
  fdisk or cfdisk to make a partition on the middle tracks or by first
  making a file (using dd) equal to half the size of the entire disk
  before creating the files that are frequently accessed, after which
  the dummy file can be deleted. Both cases assume starting from an
  empty disk.

  The latter trick is suitable for news spools where the empty directory
  structure can be placed in the middle before putting in the data
  files.  This also helps reducing fragmentation a little.

  This little trick can be used both on ordinary drives as well as RAID
  systems. In the latter case the calculation for centring the tracks
  will be different, if possible. Consult the latest RAID manual.

  The speed difference this makes depends on the drives, but a 50
  percent improvement is a typical value.


  6.8.1.  Disk Speed Values

  The same mechanical head disk assembly (HDA) is often available with a
  number of interfaces (IDE, SCSI etc) and the mechanical parameters are
  therefore often comparable. The mechanics is today often the limiting
  factor but development is improving things steadily. There are two
  main parameters, usually quoted in milliseconds (ms):


  ╖  Head movement - the speed at which the read-write head is able to
     move from one track to the next, called access time.  If you do the
     mathematics and doubly integrate the seek first across all possible
     starting tracks and then across all possible target tracks you will
     find that this is equivalent of a stroke across a third of all
     tracks.

  ╖  Rotational speed - which determines the time taken to get to the
     right sector, called latency.

  After voice coils replaced stepper motors for the head movement the
  improvements seem to have levelled off and more energy is now spent
  (literally) at improving rotational speed. This has the secondary
  benefit of also improving transfer rates.

  Some typical values:



                                Drive type


       Access time (ms)        | Fast  Typical   Old
       ---------------------------------------------
       Tract-to-track             <1       2       8
       Average seek               10      15      30
       End-to-end                 10      30      70




  This shows that the very high end drives offer only marginally better
  access times then the average drives but that the old drives based on
  stepper motors are significantly worse.







  Rotational speed (RPM)  |  3600 | 4500 | 4800 | 5400 | 7200 | 10000
  -------------------------------------------------------------------
  Latency          (ms)   |    17 |   13 | 12.5 | 11.1 |  8.3 |   6.0




  As latency is the average time taken to reach a given sector, the
  formula is quite simply


       latency (ms) = 60000 / speed (RPM)




  Clearly this too is an example of diminishing returns for the efforts
  put into development. However, what really takes off here is the power
  consumption, heat and noise.



  6.9.  Stacking

  One of the advantages of a layered design of an operating system is
  that you have the flexibility to put the pieces together in a number
  of ways.  For instance you can cache a CD-ROM with cachefs that is a
  volume striped over 2 drives. This in turn can be set up translucently
  with a volume that is NFS mounted from another machine.  RAID can be
  stacked in several layers to offer very fast seek and transfer in such
  a way that it will work if even 3 drives fail.  The choices are many,
  limited only by imagination and, probably more importantly, money.



  6.10.  Recommendations

  There is a near infinite number of combinations available but my
  recommendation is to start off with a simple setup without any fancy
  add-ons. Get a feel for what is needed, where the maximum performance
  is required, if it is access time or transfer speed that is the bottle
  neck, and so on. Then phase in each component in turn. As you can
  stack quite freely you should be able to retrofit most components in
  as time goes by with relatively few difficulties.

  RAID is usually a good idea but make sure you have a thorough grasp of
  the technology and a solid back up system.




  7.  Other Operating Systems

  Many Linux users have several operating systems installed, often
  necessitated by hardware setup systems that run under other operating
  systems, typically DOS or some flavour of Windows. A small section on
  how best to deal with this is therefore included here.


  7.1.  DOS

  Leaving aside the debate on weather or not DOS qualifies as an
  operating system one can in general say that it has little
  sophistication with respect to disk operations. The more important
  result of this is that there can be severe difficulties in running
  various versions of DOS on large drives, and you are therefore
  strongly recommended in reading the Large Drives mini-HOWTO. One
  effect is that you are often better off placing DOS on low track
  numbers.

  Having been designed for small drives it has a rather unsophisticated
  file system (fat) which when used on large drives will allocate
  enormous block sizes. It is also prone to block fragmentation which
  will after a while cause excessive seeks and slow effective transfers.

  One solution to this is to use a defragmentation program regularly but
  it is strongly recommended to back up data and verify the disk before
  defragmenting. All versions of DOS have chkdsk that can do some disk
  checking, newer versions also have scandisk which is somewhat better.
  There are many defragmentation programs available, some versions have
  one called defrag. Norton Utilities have a large suite of disk tools
  and there are many others available too.

  As always there are snags, and this particular snake in our drive
  paradise is called hidden files. Some vendors started to use these for
  copy protection schemes and would not take kindly to being moved to a
  different place on the drive, even if it remained in the same place in
  the directory structure. The result of this was that newer
  defragmentation programs will not touch any hidden file, which in turn
  reduces the effect of defragmentation.

  Being a single tasking, single threading and single most other things
  operating system there is very little gains in using multiple drives
  unless you use a drive controller with built in RAID support of some
  kind.

  There are a few utilities called join and subst which can do some
  multiple drive configuration but there is very little gains for a lot
  of work. Some of these commands have been removed in newer versions.

  In the end there is very little you can do, but not all hope is lost.
  Many programs need fast, temporary storage, and the better behaved
  ones will look for environment variables called TMPDIR or TEMPDIR
  which you can set to point to another drive. This is often best done
  in autoexec.bat.


  ______________________________________________________________________
  SET TMPDIR=E:/TMP
  SET TEMPDIR=E:/TEMP
  ______________________________________________________________________



  Not only will this possibly gain you some speed but also it can reduce
  fragmentation.

  There have been reports about difficulties in removing multiple
  primary partitions using the fdisk program that comes with DOS. Should
  this happen you can instead use a Linux rescue disk with Linux fdisk
  to repair the system.

  Don't forget there are other alternatives to DOS, the most well known
  being DR-DOS <http://www.caldera/dos/> from Caldera
  <http://www.caldera/>.  This is a direct descendant from DR-DOS from
  Digital Research.  It offers many features not found in the more
  common DOS, such as multi tasking and long filenames.

  Another alternative which also is free is Free DOS
  <http://www.freedos.org/> which is a project under development. A
  number of free utilities are also available.

  7.2.  Windows

  Most of the above points are valid for Windows too, with the exception
  of Windows95 which apparently has better disk handling, which will get
  better performance out of SCSI drives.

  A useful thing is the introduction of long filenames, to read these
  from Linux you will need the vfat file system for mounting these
  partitions.


  Disk fragmentation is still a problem. Some of this can be avoided by
  doing a defragmentation immediately before and immediately after
  installing large programs or systems. I use this scheme at work and
  have found it to work quite well. Purging unused files and emptying
  the waste basket first can improve defragmentation further.

  Windows also use swap drives, redirecting this to another drive can
  give you some performance gains. There are several mini-HOWTOs telling
  you how best to share swap space between various operating systems.




  The trick of setting TEMPDIR can still be used but not all programs
  will honour this setting. Some do, though. To get a good overview of
  the settings in the control files you can run sysedit which will open
  a number of files for editing, one of which is the autoexec file where
  you can add the TEMPDIR settings.

  Much of the temporary files are located in the /windows/temp directory
  and changing this is more tricky. To achieve this you can use regedit
  which is rather powerful and quite capable of rendering your system in
  a state you will not enjoy, or more precisely, in a state much less
  enjoyable than windows in general.  Registry database error is a
  message that means seriously bad news.  Also you will see that many
  programs have their own private temporary directories scattered around
  the system.

  Setting the swap file to a separate partition is a better idea and
  much less risky. Keep in mind that this partition cannot be used for
  anything else, even if there should appear to be space left there.

  It is now possible to read ext2fs partitions from Windows, either by
  mounting the partition using FSDEXT2 <http://www.yipton.demon.co.uk/>
  or by using a file explorer like tool called Explore2fs
  <http://uranus.it.swin.edu.au/~jn/linux/Explore2fs.html>.



  7.3.  OS/2

  The only special note here is that you can get a file system driver
  for OS/2 that can read an ext2fs partition.


  7.4.  NT

  This is a more serious system featuring most buzzwords known to
  marketing.  It is well worth noting that it features software striping
  and other more sophisticated setups. Check out the drive manager in
  the control panel.  I do not have easy access to NT, more details on
  this can take a bit of time.

  One important snag was recently reported by acahalan at cs.uml.edu :
  (reformatted from a Usenet News posting)
  NT DiskManager has a serious bug that can corrupt your disk when you
  have several (more than one?) extended partitions.  Microsoft provides
  an emergency fix program at their web site. See the knowledge base
  <http://www.microsoft.com/kb/> for more.  (This affects Linux users,
  because Linux users have extra partitions)

  You can now read ext2fs partitions from NT using Explore2fs
  <http://uranus.it.swin.edu.au/~jn/linux/Explore2fs.html>.



  7.5.  Sun OS

  There is a little bit of confusion in this area between Sun OS vs.
  Solaris.  Strictly speaking Solaris is just Sun OS 5.x packaged with
  Openwindows and a few other things. If you run Solaris, just type
  uname -a to see your version. Parts of the reason for this confusion
  is that Sun Microsystems used to use an OS from the BSD family,
  albeight with a few bits and pieces from elsewhere as well as things
  made by themselves. This was the situation up to Sun OS 4.x.y when
  they did a "strategic roadmap decision" and decided to switch over to
  the official Unix, System V, Release 4 (aka SVR5), and Sun OS 5 was
  created.  This made a lot of people unhappy. Also this was bundled
  with other things and marketed under the name Solaris, which currently
  stands at release 7 which just recently replaced version 2.6 as the
  latest and greatest.  In spite of the large jump in version number
  this is actually a minor technical upgrade but a giant leap for
  marketing.




  7.5.1.  Sun OS 4

  This is quite familiar to most Linux users.  The last release is 4.1.4
  plus various patches.  Note however that the file system structure is
  quite different and does not conform to FSSTND so any planning must be
  based on the traditional structure. You can get some information by
  the man page on this: man hier. This is, like most man pages, rather
  brief but should give you a good start. If you are still confused by
  the structure it will at least be at a higher level.


  7.5.2.  Sun OS 5 (aka Solaris)

  This comes with a snazzy installation system that runs under
  Openwindows, it will help you in partitioning and formatting the
  drives before installing the system from CD-ROM. It will also fail if
  your drive setup is too far out, and as it takes a complete
  installation run from a full CD-ROM in a 1x only drive this failure
  will dawn on you after too long time. That is the experience we had
  where I used to work. Instead we installed everything onto one drive
  and then moved directories across.

  The default settings are sensible for most things, yet there remains a
  little oddity: swap drives. Even though the official manual recommends
  multiple swap drives (which are used in a similar fashion as on Linux)
  the default is to use only a single drive. It is recommended to change
  this as soon as possible.

  Sun OS 5 offers also a file system especially designed for temporary
  files, tmpfs. It offers significant speed improvements over ufs but
  does not survive rebooting.



  The only comment so far is: beware! Under Solaris 2.0 it seem that
  creating too big files in /tmp can cause a out of swap space kernel
  panic trap. As the evidence of what has happened is as lost as any
  data on a RAMdisk after powering down it can be hard to find out what
  has happened. What is worse, it seems that user space processes can
  cause this kernel panic and unless this problem is taken care of it is
  best not to use tmpfs in potentially hostile environments.

  Also see the notes on ``tmpfs''.

  Trivia: There is a movie also called Solaris, a science fiction movie
  that is very, very long, slow and incomprehensible. This was often
  pointed out at the time Solaris (the OS) appeared...



  7.5.3.  BeOS

  This operating system is one of the more recent one to arrive and it
  features a file system that has some database like features.

  There is a BFS file system driver being developed for Linux and is
  available in alpha stage. For more information check the Linux BFS
  page <http://hp.vector.co.jp/authors/VA008030/bfs> where patches also
  are available.




  8.  Clusters

  In this section I will briefly touch on the ways machines can be
  connected together but this is so big a topic it could be a separate
  HOWTO in its own right, hint, hint. Also, strictly speaking, this
  section lies outside the scope of this HOWTO, so if you feel like
  getting fame etc. you could contact me and take over this part and
  turn it into a new document.

  These days computers gets outdated at an incredible rate. There is
  however no reason why old hardware could not be put to good use with
  Linux. Using an old and otherwise outdated computer as a network
  server can be both useful in its own right as well as a valuable
  educational exercise. Such a local networked cluster of computers can
  take on many forms but to remain within the charter of this HOWTO I
  will limit myself to the disk strategies.  Nevertheless I would hope
  someone else could take on this topic and turn it into a document on
  its own.

  This is an exciting area of activity today, and many forms of
  clustering is available today, ranging from automatic workload
  balancing over local network to more exotic hardware such as Scalable
  Coherent Interface (SCI) which gives a tight integration of machines,
  effectively turning them into a single machine. Various kinds of
  clustering has been available for larger machines for some time and
  the VAXcluster is perhaps a well known example of this. Clustering is
  done usually in order to share resources such as disk drives, printers
  and terminals etc, but also processing resources equally transparently
  between the computational nodes.

  There is no universal definition of clustering, in here it is taken to
  mean a network of machines that combine their resources to serve
  users. Admittedly this is a rather loose definition but this will
  change later.

  These days also Linux offers some clustering features but for a
  starter I will just describe a simple local network. It is a good way
  of putting old and otherwise unusable hardware to good use, as long as
  they can run Linux or something similar.

  One of the best ways of using an old machine is as a network server in
  which case the effective speed is more likely to be limited by network
  bandwidth rather than pure computational performance. For home use you
  can move the following functionality off to an older machine used as a
  server:

  ╖  news

  ╖  mail

  ╖  web proxy

  ╖  printer server

  ╖  modem server (PPP, SLIP, FAX, Voice mail)

  You can also NFS mount drives from the server onto your workstation
  thereby reducing drive space requirements. Still read the FSSTND to
  see what directories should not be exported. The best candidates for
  exporting to all machines are /usr and /var/spool and possibly
  /usr/local but probably not /var/spool/lpd.

  Most of the time even slow disks will deliver sufficient performance.
  On the other hand, if you do processing directly on the disks on the
  server or have very fast networking, you might want to rethink your
  strategy and use faster drives. Searching features on a web server or
  news database searches are two examples of this.

  Such a network can be an excellent way of learning system
  administration and building up your own toaster network, as it often
  is called. You can get more information on this in other HOWTOs but
  there are two important things you should keep in mind:

  ╖  Do not pull IP numbers out of thin air. Configure your inside net
     using IP numbers reserved for private use, and use your network
     server as a router that handles this IP masquerading.

  ╖  Remember that if you additionally configure the router as a
     firewall you might not be able to get to your own data from the
     outside, depending on the firewall configuration.

  The nyx network provides an example of a cluster in the sense defined
  here.  It consists of the following machines:

     nyx
        is one of the two user login machines and also provides some of
        the networking services.

     nox
        (aka nyx10) is the main user login machine and is also the mail
        server.

     noc
        is a dedicated news server. The news spool is made accessible
        through NFS mounting to nyx and nox.

     arachne
        (aka www) is the web server. Web pages are written by NFS
        mounting onto nox.

  There are also some more advanced clustering projects going, notably


  ╖  The Beowulf Project
     <http://cesdis.gsfc.nasa.gov/linux/beowulf/beowulf.html>

  ╖  The Genoa Active Message Machine (GAMMA)
     <http://www.disi.unige.it/project/gamma/>


  High-tech clustering requires high-tech interconnect, and SCI is one
  of them.  To find out more you can either look up the home page of
  Dolphin Interconnect Solutions <http://www.dolphinics.no/> which is
  one of the main actors in this field, or you can have a look at scizzl
  <http://www.scizzl.com/>.


  Centralised mail servers using IMAP are becoming more and more popular
  as disks become large enough to keep all mail stored indefinitely and
  also cheap enough to make it a feasible option.  Unfortunately it has
  become clear that NFS mounting the mail archives from another machine
  can cause corruption of the IMAP database as the server software does
  not handle NFS timeouts too well, and NFS timeouts are a rather common
  occurrence.  Keep therefore the mail archive local to the IMAP server.





  9.  Mount Points

  In designing the disk layout it is important not to split off the
  directory tree structure at the wrong points, hence this section.  As
  it is highly dependent on the FSSTND it has been put aside in a
  separate section, and will most likely have to be totally rewritten
  when FHS is adopted in a Linux distribution.  In the meanwhile this
  will do.

  Remember that this is a list of where a separation can take place, not
  where it has to be. As always, good judgement is always required.

  Again only a rough indication can be given here. The values indicate



       0=don't separate here
       1=not recommended
        ...
       4=useful
       5=recommended




  In order to keep the list short, the uninteresting parts are removed.














  Directory   Suitability
  /
  |
  +-bin       0
  +-boot      0
  +-dev       0
  +-etc       0
  +-home      5
  +-lib       0
  +-mnt       0
  +-proc      0
  +-root      0
  +-sbin      0
  +-tmp       5
  +-usr       5
  | \
  | +-X11R6     3
  | +-bin       3
  | +-lib       4
  | +-local     4
  | | \
  | | +bin        2
  | | +lib        4
  | +-src       3
  |
  +-var       5
    \
    +-adm       0
    +-lib       2
    +-lock      1
    +-log       0
    +-preserve  1
    +-run       1
    +-spool     4
    | \
    | +-mail      3
    | +-mqueue    3
    | +-news      5
    | +-smail     3
    | +-uucp      3
    +-tmp       5





  There is of course plenty of adjustments possible, for instance a home
  user would not bother with splitting off the /var/spool hierarchy but
  a serious ISP should. The key here is usage.

  QUIZ! Why should /etc never be on a separate partition?  Answer:
  Mounting instructions during boot is found in the file /etc/fstab so
  if this is on a separate and unmounted partition it is like the key to
  a locked drawer is inside that drawer, a hopeless situation. (Yes,
  I'll do nearly anything to liven up this HOWTO.)





  10.  Considerations and Dimensioning

  The starting point in this will be to consider where you are and what
  you want to do. The typical home system starts out with existing
  hardware and the newly converted Linux user will want to get the most
  out of existing hardware. Someone setting up a new system for a
  specific purpose (such as an Internet provider) will instead have to
  consider what the goal is and buy accordingly. Being ambitious I will
  try to cover the entire range.

  Various purposes will also have different requirements regarding file
  system placement on the drives, a large multiuser machine would
  probably be best off with the /home directory on a separate disk, just
  to give an example.

  In general, for performance it is advantageous to split most things
  over as many disks as possible but there is a limited number of
  devices that can live on a SCSI bus and cost is naturally also a
  factor. Equally important, file system maintenance becomes more
  complicated as the number of partitions and physical drives increases.


  10.1.  Home Systems

  With the cheap hardware available today it is possible to have quite a
  big system at home that is still cheap, systems that rival major
  servers of yesteryear. While many started out with old, discarded
  disks to build a Linux server (which is how this HOWTO came into
  existence), many can now afford to buy 20 GB disks up front.

  Size remains important for some, and here are a few guidelines:


     Testing
        Linux is simple and you don't even need a hard disk to try it
        out, if you can get the boot floppies to work you are likely to
        get it to work on your hardware. If the standard kernel does not
        work for you, do not forget that often there can be special boot
        disk versions available for unusual hardware combinations that
        can solve your initial problems until you can compile your own
        kernel.


     Learning
        about operating system is something Linux excels in, there is
        plenty of documentation and the source is available. A single
        drive with 50 MB is enough to get you started with a shell, a
        few of the most frequently used commands and utilities.


     Hobby
        use or more serious learning requires more commands and
        utilities but a single drive is still all it takes, 500 MB
        should give you plenty of room, also for sources and
        documentation.


     Serious
        software development or just serious hobby work requires even
        more space. At this stage you have probably a mail and news feed
        that requires spool files and plenty of space. Separate drives
        for various tasks will begin to show a benefit. At this stage
        you have probably already gotten hold of a few drives too. Drive
        requirements gets harder to estimate but I would expect 2-4 GB
        to be plenty, even for a small server.


     Servers
        come in many flavours, ranging from mail servers to full sized
        ISP servers. A base of 2 GB for the main system should be
        sufficient, then add space and perhaps also drives for separate
        features you will offer. Cost is the main limiting factor here
        but be prepared to spend a bit if you wish to justify the "S" in
        ISP. Admittedly, not all do it.

        Basically a server is dimensioned like any machine for serious
        use with added space for the services offered, and tends to be
        IO bound rather than CPU bound.

        With cheap networking technology both for land lines as well as
        through radio nets, it is quite likely that very soon home users
        will have their own servers more or less permanently hooked onto
        the net.



  10.2.  Servers

  Big tasks require big drives and a separate section here. If possible
  keep as much as possible on separate drives. Some of the appendices
  detail the setup of a small departmental server for 10-100 users. Here
  I will present a few consideration for the higher end servers. In
  general you should not be afraid of using RAID, not only because it is
  fast and safe but also because it can make growth a little less
  painful. All the notes below come as additions to the points mentioned
  earlier.

  Popular servers rarely just happens, rather they grow over time and
  this demands both generous amounts of disk space as well as a good net
  connection.  In many of these cases it might be a good idea to reserve
  entire SCSI drives, in singles or as arrays, for each task. This way
  you can move the data should the computer fail. Note that transferring
  drives across computers is not simple and might not always work,
  especially in the case of IDE drives. Drive arrays require careful
  setup in order to reconstruct the data correctly, so you might want to
  keep a paper copy of your fstab file as well as a note of SCSI IDs.


  10.2.1.  Home Directories

  Estimate how many drives you will need, if this is more than 2 I would
  recommend RAID, strongly. If not you should separate users across your
  drives dedicated to users based on some kind of simple hashing
  algorithm.  For instance you could use the first 2 letters in the user
  name, so jbloggs is put on /u/j/b/jbloggs where /u/j is a symbolic
  link to a physical drive so you can get a balanced load on your
  drives.


  10.2.2.  Anonymous FTP

  This is an essential service if you are serious about service. Good
  servers are well maintained, documented, kept up to date, and
  immensely popular no matter where in the world they are located. The
  big server ftp.funet.fi <ftp://ftp.funet.fi> is an excellent example
  of this.

  In general this is not a question of CPU but of network bandwidth.
  Size is hard to estimate, mainly it is a question of ambition and
  service attitudes. I believe the big archive at ftp.cdrom.com
  <ftp://ftp.cdrom.com> is a *BSD machine with 50 GB disk. Also memory
  is important for a dedicated FTP server, about 256 MB RAM would be
  sufficient for a very big server, whereas smaller servers can get the
  job done well with 64 MB RAM.  Network connections would still be the
  most important factor.



  10.2.3.  WWW

  For many this is the main reason to get onto the Internet, in fact
  many now seem to equate the two. In addition to being network
  intensive there is also a fair bit of drive activity related to this,
  mainly regarding the caches. Keeping the cache on a separate, fast
  drive would be beneficial. Even better would be installing a caching
  proxy server. This way you can reduce the cache size for each user and
  speed up the service while at the same time cut down on the bandwidth
  requirements.

  With a caching proxy server you need a fast set of drives, RAID0 would
  be ideal as reliability is not important here. Higher capacity is
  better but about 2 GB should be sufficient for most. Remember to match
  the cache period to the capacity and demand. Too long periods would on
  the other hand be a disadvantage, if possible try to adjust based on
  the URL. For more information check up on the most used servers such
  as Harvest, Squid <http://www.nlanr.net/Squid> and the one from
  Netscape <http://www.netscape.com>.


  10.2.4.  Mail

  Handling mail is something most machines do to some extent. The big
  mail servers, however, come into a class of their own. This is a
  demanding task and a big server can be slow even when connected to
  fast drives and a good net feed. In the Linux world the big server at
  vger.rutgers.edu is a well known example. Unlike a news service which
  is distributed and which can partially reconstruct the spool using
  other machines as a feed, the mail servers are centralised. This makes
  safety much more important, so for a major server you should consider
  a RAID solution with emphasize on reliability. Size is hard to
  estimate, it all depends on how many lists you run as well as how many
  subscribers you have.

  Note that these days more and more switch over from using POP to pull
  mail to local machine from mail server and instead use IMAP to serve
  mail while keeping the mail archive centralised.  This means that mail
  is no longer spooled in its original sense but often builds up,
  requiring huge disk space. Also more and more (ab)use mail attachments
  to send all sorts of things across, even a small word processor
  document can easily end up over 1 MB. Size your disks generously and
  keep an eye on how much space is left.



  10.2.5.  News

  This is definitely a high volume task, and very dependent on what news
  groups you subscribe to. On Nyx there is a fairly complete feed and
  the spool files consume about 17 GB. The biggest groups are no doubt
  in the alt.binary.* hierarchy, so if you for some reason decide not to
  get these you can get a good service with perhaps 12 GB. Still others,
  that shall remain nameless, feel 2 GB is sufficient to claim ISP
  status.  In this case news expires so fast I feel the spelling IsP is
  barely justified. A full newsfeed means a traffic of a few GB every
  day and this is an ever growing number.



  10.2.6.  Others

  There are many services available on the net and even though many have
  been put somewhat in the shadows by the web. Nevertheless, services
  like archie, gopher and wais just to name a few, still exist and
  remain valuable tools on the net. If you are serious about starting a
  major server you should also consider these services. Determining the
  required volumes is hard, it all depends on popularity and demand.
  Providing good service inevitably has its costs, disk space is just
  one of them.



  10.2.7.  Server Recommendations

  Servers today require large numbers of large disks to function
  satisfactorily in commercial settings. As mean time between failure
  (MTBF) decreases rapidly as the number of components increase it is
  advisable to look into using RAID for protection and use a number of
  medium sized drives rather than one single huge disk. Also look into
  the High Availability (HA) project for more information.




  10.3.  Pitfalls

  The dangers of splitting up everything into separate partitions are
  briefly mentioned in the section about volume management. Still,
  several people have asked me to emphasize this point more strongly:
  when one partition fills up it cannot grow any further, no matter if
  there is plenty of space in other partitions.

  In particular look out for explosive growth in the news spool
  (/var/spool/news). For multi user machines with quotas keep an eye on
  /tmp and /var/tmp as some people try to hide their files there, just
  look out for filenames ending in gif or jpeg...

  In fact, for single physical drives this scheme offers very little
  gains at all, other than making file growth monitoring easier (using
  'df') and physical track positioning. Most importantly there is no
  scope for parallel disk access. A freely available volume management
  system would solve this but this is still some time in the future.
  However, when more specialised file systems become available even a
  single disk could benefit from being divided into several partitions.




  11.  Disk Layout

  With all this in mind we are now ready to embark on the layout. I have
  based this on my own method developed when I got hold of 3 old SCSI
  disks and boggled over the possibilities.

  The tables in the appendices are designed to simplify the mapping
  process. They have been designed to help you go through the process of
  optimizations as well as making an useful log in case of system
  repair. A few examples are also given.



  11.1.  Selection for Partitioning

  Determine your needs and set up a list of all the parts of the file
  system you want to be on separate partitions and sort them in
  descending order of speed requirement and how much space you want to
  give each partition.

  The table in ``Appendix A'' section is a useful tool to select what
  directories you should put on different partitions. It is sorted in a
  logical order with space for your own additions and notes about
  mounting points and additional systems. It is therefore NOT sorted in
  order of speed, instead the speed requirements are indicated by
  bullets ('o').

  If you plan to RAID make a note of the disks you want to use and what
  partitions you want to RAID. Remember various RAID solutions offers
  different speeds and degrees of reliability.

  (Just to make it simple I'll assume we have a set of identical SCSI
  disks and no RAID)



  11.2.  Mapping Partitions to Drives

  Then we want to place the partitions onto physical disks. The point of
  the following algorithm is to maximise parallelizing and bus capacity.
  In this example the drives are A, B and C and the partitions are
  987654321 where 9 is the partition with the highest speed requirement.
  Starting at one drive we 'meander' the partition line over and over
  the drives in this way:



               A : 9 4 3
               B : 8 5 2
               C : 7 6 1




  This makes the 'sum of speed requirements' the most equal across each
  drive.

  Use the table in ``Appendix B'' section to select what drives to use
  for each partition in order to optimize for paralellicity.

  Note the speed characteristics of your drives and note each directory
  under the appropriate column. Be prepared to shuffle directories,
  partitions and drives around a few times before you are satisfied.


  11.3.  Sorting Partitions on Drives

  After that it is recommended to select partition numbering for each
  drive.

  Use the table in ``Appendix C'' section to select partition numbers in
  order to optimize for track characteristics.  At the end of this you
  should have a table sorted in ascending partition number. Fill these
  numbers back into the tables in appendix A and B.

  You will find these tables useful when running the partitioning
  program (fdisk or cfdisk) and when doing the installation.



  11.4.  Optimizing

  After this there are usually a few partitions that have to be
  'shuffled' over the drives either to make them fit or if there are
  special considerations regarding speed, reliability, special file
  systems etc. Nevertheless this gives what this author believes is a
  good starting point for the complete setup of the drives and the
  partitions. In the end it is actual use that will determine the real
  needs after we have made so many assumptions. After commencing
  operations one should assume a time comes when a repartitioning will
  be beneficial.

  For instance if one of the 3 drives in the above mentioned example is
  very slow compared to the two others a better plan would be as
  follows:



               A : 9 6 5
               B : 8 7 4
               C : 3 2 1






  11.4.1.  Optimizing by Characteristics

  Often drives can be similar in apparent overall speed but some
  advantage can be gained by matching drives to the file size
  distribution and frequency of access. Thus binaries are suited to
  drives with fast access that offer command queueing, and libraries are
  better suited to drives with larger transfer speeds where IDE offers
  good performance for the money.



  11.4.2.  Optimizing by Drive Parallelising

  Avoid drive contention by looking at tasks: for instance if you are
  accessing /usr/local/bin chances are you will soon also need files
  from /usr/local/lib so placing these at separate drives allows less
  seeking and possible parallel operation and drive caching. It is quite
  possible that choosing what may appear less than ideal drive
  characteristics will still be advantageous if you can gain parallel
  operations. Identify common tasks, what partitions they use and try to
  keep these on separate physical drives.

  Just to illustrate my point I will give a few examples of task
  analysis here.



     Office software
        such as editing, word processing and spreadsheets are typical
        examples of low intensity software both in terms of CPU and disk
        intensity. However, should you have a single server for a huge
        number of users you should not forget that most such software
        have auto save facilities which cause extra traffic, usually on
        the home directories. Splitting users over several drives would
        reduce contention.


     News
        readers also feature auto save features on home directories so
        ISPs should consider separating home directories

        News spools are notorious for their deeply nested directories
        and their large number of very small files. Loss of a news spool
        partition is not a big problem for most people, too, so they are
        good candidates for a RAID 0 setup with many small disks to
        distribute the many seeks among multiple spindles. It is
        recommended in the manuals and FAQs for the INN news server to
        put news spool and .overview files on separate drives for larger
        installations.

        There is also a web page dedicated to INN optimising
        <http://www.spinne.com/usenet/inn-perf.html> well worth reading.



     Database
        applications can be demanding both in terms of drive usage and
        speed requirements. The details are naturally application
        specific, read the documentation carefully with disk
        requirements in mind. Also consider RAID both for performance
        and reliability.


     E-mail
        reading and sending involves home directories as well as in- and
        outgoing spool files. If possible keep home directories and
        spool files on separate drives. If you are a mail server or a
        mail hub consider putting in- and outgoing spool directories on
        separate drives.

        Losing mail is an extremely bad thing, if you are managing an
        ISP or major hub. Think about RAIDing your mail spool and
        consider frequent backups.


     Software development
        can require a large number of directories for binaries,
        libraries, include files as well as source and project files. If
        possible split as much as possible across separate drives. On
        small systems you can place /usr/src and project files on the
        same drive as the home directories.


     Web browsing
        is becoming more and more popular. Many browsers have a local
        cache which can expand to rather large volumes. As this is used
        when reloading pages or returning to the previous page, speed is
        quite important here. If however you are connected via a well
        configured proxy server you do not need more than typically a
        few megabytes per user for a session.  See also the sections on
        ``Home Directories'' and ``WWW''.





  11.5.  Compromises

  One way to avoid the aforementioned pitfalls is to only set off fixed
  partitions to directories with a fairly well known size such as swap,
  /tmp and /var/tmp and group together the remainders into the remaining
  partitions using symbolic links.

  Example: a slow disk (slowdisk), a fast disk (fastdisk) and an
  assortment of files. Having set up swap and tmp on fastdisk; and /home
  and root on slowdisk we have (the fictitious) directories /a/slow,
  /a/fast, /b/slow and /b/fast left to allocate on the partitions
  /mnt.slowdisk and /mnt.fastdisk which represents the remaining
  partitions of the two drives.

  Putting /a or /b directly on either drive gives the same properties to
  the subdirectories. We could make all 4 directories separate
  partitions but would lose some flexibility in managing the size of
  each directory. A better solution is to make these 4 directories
  symbolic links to appropriate directories on the respective drives.

  Thus we make



       /a/fast point to /mnt.fastdisk/a/fast   or   /mnt.fastdisk/a.fast
       /a/slow point to /mnt.slowdisk/a/slow   or   /mnt.slowdisk/a.slow
       /b/fast point to /mnt.fastdisk/b/fast   or   /mnt.fastdisk/b.fast
       /b/slow point to /mnt.slowdisk/b/slow   or   /mnt.slowdisk/b.slow




  and we get all fast directories on the fast drive without having to
  set up a partition for all 4 directories. The second (right hand)
  alternative gives us a flatter files system which in this case can
  make it simpler to keep an overview of the structure.

  The disadvantage is that it is a complicated scheme to set up and plan
  in the first place and that all mount points and partitions have to be
  defined before the system installation.




  12.  Implementation

  Having done the layout you should now have a detailled description on
  what goes where. Most likely this will be on paper but hopefully
  someone will make a more automated system that can deal with
  everything from the design, through partitioning to formatting and
  installation. This is the route one will have to take to realise the
  design.

  Modern distributions come with installation tools that will guide you
  through partitioning and formatting and also set up /etc/fstab for you
  automatically. For later modifications, however, you will need to
  understand the underlying mechanisms.



  12.1.  Drives and Partitions

  When you start DOS or the like you will find all partitions labeled C:
  and onwards, with no differentiation on IDE, SCSI, network or whatever
  type of media you have. In the world of Linux this is rather
  different. During booting you will see partitions described like this:

  ______________________________________________________________________
  Dec  6 23:45:18 demos kernel: Partition check:
  Dec  6 23:45:18 demos kernel:  sda: sda1
  Dec  6 23:45:18 demos kernel:  hda: hda1 hda2
  ______________________________________________________________________



  SCSI drives are labelled sda, sdb, sdc etc, and (E)IDE drives are
  labelled hda, hdb, hdc etc.  There are also standard names for all
  devices, full information can be found in /dev/MAKEDEV and
  /usr/src/linux/Documentation/devices.txt.

  Partitions are labelled numerically for each drive hda1, hda2 and so
  on.  On SCSI drives there can be 15 partitions per drive, on EIDE
  drives there can be 63 partitions per drive. Both limits exceed what
  is currently useful for most disks.
  These are then mounted according to the file /etc/fstab before they
  appear as a part of the file system.



  12.2.  Partitioning

  First you have to partition each drive into a number of separate
  partitions.  Under Linux there are two main methods, fdisk and the
  more screen oriented cfdisk. These are complex programs, read the
  manual very carefully.


  Partitions come in 3 flavours, primary, extended and logical.  You
  have to use primary partitions for booting, but there is a maximum of
  4 primary partitions. If you want more you have to define an extended
  partition within which you define your logical partitions.

  Each partition has an identifier number which tells the operating
  system what it is, for Linux the types swap(82) and ext2fs(83) are the
  ones you will need to know.

  There is a readme file that comes with fdisk that gives more in-depth
  information on partitioning.

  Someone has just made a Partitioning HOWTO which contains excellent,
  in depth information on the nitty-gritty of partitioning. Rather than
  repeating it here and bloating this document further, I will instead
  refer you to it instead.

  Redhat has written a screen oriented utility called Disk Druid which
  is supposed to be a user friendly alternative to fdisk and cfdisk and
  also automates a few other things. Unfortunately this product is not
  quite mature so if you use it and cannot get it to work you are well
  advised to try fdisk or cfdisk.

  The Ranish Partition Manager
  <http://www.users.intercom.com/~ranish/part/> is another free
  alternative, while Partition Magic <http://www.powerquest.com> is a
  popular commercial alternative which also offers some support for
  resizing ext2fs partitions.

  Note that Windows will complain if it finds more than one primary
  partition on a drive.  Also it appears to assign drive letters to
  primary partitions as it finds disks before starting over from the
  first disk to assign subsequent drive names to logical partitions.

  If you want DOS/Windows on your system you should make that partition
  first, a primary one to boot to, made with the DOS fdisk program.
  Then if you want NT you put that one in.  Finally, for Linux, you
  create those partitions with the Linux fdisk program or equivalents.
  Linux is flexible enough to boot from both primary as well as logical
  partitions.



  12.3.  Repartitioning

  Sometimes it is necessary to change the sizes of existing partitions
  while keeping the contents intact. One way is of course to back up
  everything, recreate new partitions and then restore the old contents,
  and while this gives your back up system a good test it is also rather
  time consuming.

  Partition resizing is a simpler alternative where a file system is
  first shrunk to desired volume and then the partition table is updated
  to reflect the new end of partition position. This process is
  therefore very file system sensitive.

  Repartitioning requires there to be free space at the end of the file
  space so to ensure you are able to shrink the size you should first
  defragment your drive and empty any wastebaskets.

  Using fips <http://www.igd.fgh.de/~aschaefe/fips/> you can resize a
  fat partition, and the latest version 1.6 of fips or fips 2.0 are also
  able to resize fat32 partition.  Note that these programs actually run
  under DOS.

  Resizing other file systems are much more complicated but one popular
  commercial system Partition Magic <http://www.powerquest.com> is able
  to resize more file system types, including ext2fs using the resize2fs
  program.


  In order to get the most out of fips you should first delete
  unnecessary files, empty wastebaskets etc.  before defragmenting your
  drive.  This way you can allocate more space to other partitions.  Id
  the program complains there are still files at the end of your drive
  it is probably hidden files generated by Microsoft Mirror or Norton
  Image.  These are probably called image.idx and image.dat and contain
  backups of some system files.

  There are reports that in some Windows defragmentation programs you
  should make sure the box "allow Windows to move files around" is not
  checked, otherwise you will end up with some files in the last
  cylinder of the partition which will prevent FIPS from reclaiming
  space.

  If you still have unmovable files at the end of your DOS partition you
  should get the DOS program "showfat" version 3.0 or higher.  This
  shows you what files are where so you can deal with them directly.

  Repartitioning is as dangerous process as any other partitioning so
  you are advised to have a fresh backup handy.



  12.4.  Microsoft Partition Bug

  In Microsoft products all the way up to Win 98 there is a tricky bug
  that can cause you a bit of trouble: if you have several primary fat
  partitions and the last extended partition is not a fat partition the
  Microsoft system will try to mount the last partition as if it were a
  FAT partition in place of the last primary FAT partition.

  There is more information <http://www.v-com.com/95Notes.html>
  available on the net on this.

  To avoid this you can place a small logical fat partition at the very
  end of your disk.

  Since some hardware comes with setup software that is available under
  DOS only this could come in handy anyway. Notable examples are RAID
  controllers from DPT and a number of networking cards.


  12.5.  Multiple Devices ( md )

  Being in a state of flux you should make sure to read the latest
  documentation on this kernel feature. It is not yet stable, beware.


  Briefly explained it works by adding partitions together into new
  devices md0, md1 etc. using mdadd before you activate them using
  mdrun. This process can be automated using the file /etc/mdtab.

  The latest md system uses a /etc/raidtab and a different syntax. Make
  sure your RAID-tools package matches the md version as the internal
  protocol has changed.

  Then you then treat these like any other partition on a drive. Proceed
  with formatting etc. as described below using these new devices.

  There is now also a HOWTO in development for RAID using md you should
  read.



  12.6.  Formatting

  Next comes partition formatting, putting down the data structures that
  will describe the files and where they are located. If this is the
  first time it is recommended you use formatting with verify. Strictly
  speaking it should not be necessary but this exercises the I/O hard
  enough that it can uncover potential problems, such as incorrect
  termination, before you store your precious data. Look up the command
  mkfs for more details.

  Linux can support a great number of file systems, rather than
  repeating the details you can read the man page for fs which describes
  them in some details. Note that your kernel has to have the drivers
  compiled in or made as modules in order to be able to use these
  features. When the time comes for kernel compiling you should read
  carefully through the file system feature list. If you use make
  menuconfig you can get online help for each file system type.

  Note that some rescue disk systems require minix, msdos and ext2fs to
  be compiled into the kernel.

  Also swap partitions have to be prepared, and for this you use mkswap.



  12.7.  Mounting

  Data on a partition is not available to the file system until it is
  mounted on a mount point. This can be done manually using mount or
  automatically during booting by adding appropriate lines to
  /etc/fstab. Read the manual for mount and pay close attention to the
  tabulation.



  12.8.  fstab

  During the booting process the system mounts all partitions as
  described in the fstab file which can look something like this:




       # <file system>   <mount point>   <type>  <options>   <dump>  <pass>
       /dev/hda2          /               ext2    defaults    0       1
       None               none            swap    sw          0       0
       proc               /proc           proc    defaults    0       0
       /dev/hda1          /dosc           vfat    defaults    0       1


  This file is somewhat sensitive to the formatting used so it is best
  and also most convenient to edit it using one of the editing tools
  made for this purpose.

  Briefly, the fields are partition name, where to mount the partition,
  type of file system, when to mount, when to dump for backup and when
  to do fsck.

  Linux offers the possibility of parallel file checking (fsck) but to
  be efficient it is important not to fsck more than one partition on a
  drive at a time.

  For more information refer to the man page for mount and fstab.



  12.9.  Recommendations

  Having constructed and implemented your clever scheme you are well
  advised to make a complete record of it all, on paper.  After all
  having all the necessary information on disk is no use if the machine
  is down.

  Partition tables can be damaged or lost, in which case it is
  excruciatingly important that you enter the exact same numbers into
  fdisk so you can rescue your system.  You can use the program printpar
  to make a clear record of the tables. Also write down the SCSI numbers
  or IDE names for each disk so you can put the system together again in
  the right order.




  13.  Maintenance

  It is the duty of the system manager to keep an eye on the drives and
  partitions. Should any of the partitions overflow, the system is
  likely to stop working properly, no matter how much space is available
  on other partitions, until space is reclaimed.

  Partitions and disks are easily monitored using df and should be done
  frequently, perhaps using a cron job or some other general system
  management tool.

  Do not forget the swap partitions, these are best monitored using one
  of the memory statistics programs such as free, procinfo or top.

  Drive usage monitoring is more difficult but it is important for the
  sake of performance to avoid contention - placing too much demand on a
  single drive if others are available and idle.

  It is important when installing software packages to have a clear idea
  where the various files go. As previously mentioned GCC keeps binaries
  in a library directory and there are also other programs that for
  historical reasons are hard to figure out, X11 for instance has an
  unusually complex structure.

  When your system is about to fill up it is about time to check and
  prune old logging messages as well as hunt down core files. Proper use
  of ulimit in global shell settings can help saving you from having
  core files littered around the system.





  13.1.  Backup

  The observant reader might have noticed a few hints about the
  usefulness of making backups. Horror stories are legio about accidents
  and what happened to the person responsible when the backup turned out
  to be non-functional or even non existent. You might find it simpler
  to invest in proper backups than a second, secret identity.

  There are many options and also a mini-HOWTO ( Backup-With-MSDOS )
  detailling what you need to know. In addition to the DOS specifics it
  also contains general information and further leads.

  In addition to making these backups you should also make sure you can
  restore the data. Not all systems verify that the data written is
  correct and many administrators have started restoring the system
  after an accident happy in the belief that everything is working, only
  to discover to their horror that the backups were useless. Be careful.



  13.2.  Defragmentation

  This is very dependent on the file system design, some suffer fast and
  nearly debilitating fragmentation. Fortunately for us, ext2fs does not
  belong to this group and therefore there has been very little talk
  about defragmentation tools. It does in fact exist but is hardly ever
  needed.

  If for some reason you feel this is necessary, the quick and easy
  solution is to do a backup and a restore. If only a small area is
  affected, for instance the home directories, you could tar it over to
  a temporary area on another partition, verify the archive, delete the
  original and then untar it back again.



  13.3.  Deletions

  Quite often disk space shortages can be remedied simply by deleting
  unnecessary files that accumulate around the system. Quite often
  programs that terminate abnormally cause all kinds of mess lying
  around the oddest places. Normally a core dump results after such an
  incident and unless you are going to debug it you can simply delete
  it. These can be found everywhere so you are advised to do a global
  search for them now and then.

  Unexpected termination can also cause all sorts of temporary files
  remaining in places like /tmp or /var/tmp, files that are
  automatically removed when the program ends normally. Rebooting cleans
  up some of these areas but not necessary all and if you have a long
  uptime you could end up with a lot of old junk. If space is short you
  have to delete with care, make sure the file is not in active use
  first. Utilities like file can often tell you what kind of file you
  are looking at.

  Many things are logged when the system is running, mostly to files in
  the /var/log area. In particular the file /var/log/messages tends to
  grow until deleted. It is a good idea to keep a small archive of old
  log files around for comparison should the system start to behave
  oddly.

  If the mail or news system is not working properly you could have
  excessive growth in their spool areas, /var/spool/mail and
  /var/spool/news respectively. Beware of the overview files as these
  have a leading dot which makes them invisible to ls -l, it is always
  better to use ls -Al which will reveal them.
  User space overflow is a particularly tricky topic. Wars have been
  waged between system administrators and users. Tact, diplomacy and a
  generous budget for new drives is what is needed. Make use of the
  message-of-the-day feature, information displayed during login from
  the /etc/motd file to tell users when space is short.  Setting the
  default shell settings to prevent core files being dumped can save you
  a lot of work too.

  Certain kinds of people try to hide files around the system, usually
  trying to take advantage of the fact that files with a leading dot in
  the name are invisible to the ls command.  One common example are
  files that look like ... that normally either are not seen, or, when
  using ls -al disappear in the noise of normal files like . or .. that
  are in every directory.  There is however a countermeasure to this,
  use ls -Al that suppresses . or .. but shows all other dot-files.



  13.4.  Upgrades

  No matter how large your drives, time will come when you will find you
  need more. As technology progresses you can get ever more for your
  money. At the time of writing this, it appears that 6.4 GB drives
  gives you the most bang for your bucks.

  Note that with IDE drives you might have to remove an old drive, as
  the maximum number supported on your mother board is normally only 2
  or some times 4. With SCSI you can have up to 7 for narrow (8-bit)
  SCSI or up to 15 for wide (15 bit) SCSI, per channel. Some host
  adapters can support more than a single channel and in any case you
  can have more than one host adapter per system. My personal
  recommendation is that you will most likely be better off with SCSI in
  the long run.

  The question comes, where should you put this new drive? In many cases
  the reason for expansion is that you want a larger spool area, and in
  that case the fast, simple solution is to mount the drive somewhere
  under /var/spool. On the other hand newer drives are likely to be
  faster than older ones so in the long run you might find it worth your
  time to do a full reorganizing, possibly using your old design sheets.

  If the upgrade is forced by running out of space in partitions used
  for things like /usr or /var the upgrade is a little more involved.
  You might consider the option of a full re-installation from your
  favourite (and hopefully upgraded) distribution. In this case you will
  have to be careful not to overwrite your essential setups. Usually
  these things are in the /etc directory. Proceed with care, fresh
  backups and working rescue disks. The other possibility is to simply
  copy the old directory over to the new directory which is mounted on a
  temporary mount point, edit your /etc/fstab file, reboot with your new
  partition in place and check that it works.  Should it fail you can
  reboot with your rescue disk, re-edit /etc/fstab and try again.

  Until volume management becomes available to Linux this is both
  complicated and dangerous. Do not get too surprised if you discover
  you need to restore your system from a backup.

  The Tips-HOWTO gives the following example on how to move an entire
  directory structure across:

  ______________________________________________________________________
  (cd /source/directory; tar cf - . ) | (cd /dest/directory; tar xvfp -)
  ______________________________________________________________________



  While this approach to moving directory trees is portable among many
  Unix systems, it is inconvenient to remember. Also, it fails for
  deeply nested directory trees when pathnames become to long to handle
  for tar (GNU tar has special provisions to deal with long pathnames).

  If you have access to GNU cp (which is always the case on Linux
  systems), you could as well use


  ______________________________________________________________________
  cp -av /source/directory /dest/directory
  ______________________________________________________________________



  GNU cp knows specifically about symbolic links, FIFOs and device files
  and will copy them correctly.

  Remember that it might not be a good idea to try to transfer /dev or
  /proc.




  13.5.  Recovery

  System crashes come in many and entertaining flavours, and partition
  table corruption always guarantees plenty of excitement.  A recent and
  undoubtedly useful tool for those of us who are happy with the normal
  level of excitement, is gpart <http://www.stud.uni-
  hannover.de/user/76201/gpart> which means "Guess PC-Type hard disk
  partitions". Useful.




  14.  Advanced Issues

  Linux and related systems offer plenty of possibilities for fast,
  efficient and devastating destruction. This document is no exception.
  With power comes dangers and the following sections describe a few
  more esoteric issues that should not be attempted before reading and
  understanding the documentation, the issues and the dangers. You
  should also make a backup. Also remember to try to restore the system
  from scratch from your backup at least once.  Otherwise you might not
  be the first to be found with a perfect backup of your system and no
  tools available to reinstall it (or, even more embarrassing, some
  critical files missing on tape).

  The techniques described here are rarely necessary but can be used for
  very specific setups. Think very clearly through what you wish to
  accomplish before playing around with this.


  14.1.  Hard Disk Tuning

  The hard drive parameters can be tuned using the hdparms utility. Here
  the most interesting parameter is probably the read-ahead parameter
  which determines how much prefetch should be done in sequential
  reading.

  If you want to try this out it makes most sense to tune for the
  characteristic file size on your drive but remember that this tuning
  is for the entire drive which makes it a bit more difficult. Probably
  this is only of use on large servers using dedicated news drives etc.

  For safety the default hdparm settings are rather conservative. The
  disadvantage is that this mean you can get lost interrupts if you have
  a high frequency of IRQs as you would when using the serial port and
  an IDE disk as IRQs from the latter would mask other IRQs. This would
  be noticeable as less then ideal performance when downloading data
  from the net to disk. Setting hdparm -u1 device would prevent this
  masking and either improve your performance or, depending on hardware,
  corrupt the data on your disk. Experiment with caution and fresh
  backups.


  14.2.  File System Tuning

  Most file systems come with a tuning utility and for ext2fs there is
  the tune2fs utility. Several parameters can be modified but perhaps
  the most useful parameter here is what size should be reserved and who
  should be able to take advantage of this which could help you getting
  more useful space out of your drives, possibly at the cost of less
  room for repairing a system should it crash.


  14.3.  Spindle Synchronizing

  This should not in itself be dangerous, other than the peculiar fact
  that the exact details of the connections remain unclear for many
  drives. The theory is simple: keeping a fixed phase difference between
  the different drives in a RAID setup makes for less waiting for the
  right track to come into position for the read/write head. In practice
  it now seems that with large read-ahead buffers in the drives the
  effect is negligible.

  Spindle synchronisation should not be used on RAID0 or RAID 0/1 as you
  would then lose the benefit of having the read heads over different
  areas of the mirrored sectors.



  15.  Further Information

  There is wealth of information one should go through when setting up a
  major system, for instance for a news or general Internet service
  provider.  The FAQs in the following groups are useful:


  15.1.  News groups

  Some of the most interesting news groups are:

  ╖  Storage <news:comp.arch.storage>.

  ╖  PC storage <news:comp.sys.ibm.pc.hardware.storage>.

  ╖  AFS <news:alt.filesystems.afs>.

  ╖  SCSI <news:comp.periphs.scsi>.

  ╖  Linux setup <news:comp.os.linux.setup>.

  Most newsgroups have their own FAQ that are designed to answer most of
  your questions, as the name Frequently Asked Questions indicate. Fresh
  versions should be posted regularly to the relevant newsgroups. If you
  cannot find it in your news spool you could go directly to the FAQ
  main archive FTP site <ftp://rtfm.mit.edu>. The WWW versions can be
  browsed at FAQ main archive WWW site <http://www.cis.ohio-
  state.edu/hypertext/faq/usenet/FAQ-List.html>.

  Some FAQs have their own home site, of particular interest here are

  ╖  SCSI FAQ <http://www.paranoia.com/~filipg/HTML/LINK/F_SCSI.html>
     and

  ╖  comp.arch.storage FAQ
     <http://alumni.caltech.edu/~rdv/comp_arch_storage/FAQ-1.html>.



  15.2.  Mailing Lists

  These are low noise channels mainly for developers. Think twice before
  asking questions there as noise delays the development.  Some relevant
  lists are linux-raid, linux-scsi and linux-ext2fs.  Many of the most
  useful mailing lists run on the vger.rutgers.edu server but this is
  notoriously overloaded, so try to find a mirror. There are some lists
  mirrored at The Redhat Home Page <http://www.redhat.com>.  Many lists
  are also accessible at linuxhq <http://www.linuxhq.com/lnxlists>, and
  the rest of the web site is a gold mine of useful information.

  If you want to find out more about the lists available you can send a
  message with the line lists to the list server at vger.rutgers.edu (
  [e-mail]).  If you need help on how to use the mail
  server just send the line help to the same address.  Due to the
  popularity of this server it is likely it takes a bit to time before
  you get a reply or even get messages after you send a subscribe
  command.

  There is also a number of other majordomo list servers that can be of
  interest such as the EATA driver list ( [e-mail])
  and the Intelligent IO list [e-mail].

  Mailing lists are in a state of flux but you can find links to a
  number of interesting lists from the Linux Documentation Homepage
  <http://metalab.unc.edu/LDP/>.



  15.3.  HOWTO

  These are intended as the primary starting points to get the
  background information as well as show you how to solve a specific
  problem.  Some relevant HOWTOs are Bootdisk, Installation,  SCSI and
  UMSDOS.  The main site for these is the LDP archive
  <http://metalab.unc.edu/LDP/> at Metalab (formerly known as Sunsite).

  There is a a new HOWTO out that deals with setting up a DPT RAID
  system, check out the DPT RAID HOWTO homepage
  <http://www.ram.org/computing/linux/dpt_raid.html>.




  15.4.  Mini-HOWTO

  These are the smaller free text relatives to the HOWTOs.  Some
  relevant mini-HOWTOs are Backup-With-MSDOS, Diskless, LILO, Large
  Disk, Linux+DOS+Win95+OS2, Linux+OS2+DOS, Linux+Win95, NFS-Root,
  Win95+Win+Linux, ZIP Drive .  You can find these at the same place as
  the HOWTOs, usually in a sub directory called mini. Note that these
  are scheduled to be converted into SGML and become proper HOWTOs in
  the near future.

  The old Linux Large IDE mini-HOWTO is no longer valid, instead read
  /usr/src/linux/drivers/block/README.ide or
  /usr/src/linux/Documentation/ide.txt.


  15.5.  Local Resources

  In most distributions of Linux there is a document directory
  installed, have a look in the /usr/doc directory.  where most packages
  store their main documentation and README files etc.  Also you will
  here find the HOWTO archive ( /usr/doc/HOWTO) of ready formatted
  HOWTOs and also the mini-HOWTO archive ( /usr/doc/HOWTO/mini
  <file:///usr/doc/HOWTO/mini>) of plain text documents.

  Many of the configuration files mentioned earlier can be found in the
  /etc directory. In particular you will want to work with the
  /etc/fstab file that sets up the mounting of partitions and possibly
  also /etc/mdtab file that is used for the md system to set up RAID.

  The kernel source in /usr/src/linux <file:///usr/src/linux> is, of
  course, the ultimate documentation. In other words, use the source,
  Luke.  It should also be pointed out that the kernel comes not only
  with source code which is even commented (well, partially at least)
  but also an informative documentation directory
  <file:///usr/src/linux/Documentation>.  If you are about to ask any
  questions about the kernel you should read this first, it will save
  you and many others a lot of time and possibly embarrassment.

  Also have a look in your system log file ( /var/log/messages) to see
  what is going on and in particular how the booting went if too much
  scrolled off your screen. Using tail -f /var/log/messages in a
  separate window or screen will give you a continuous update of what is
  going on in your system.

  You can also take advantage of the /proc file system that is a window
  into the inner workings of your system.  Use cat rather than more to
  view the files as they are reported as being zero length.




  15.6.  Web Pages

  There is a huge number of informative web pages out there and by their
  very nature they change quickly so don't be too surprised if these
  links become quickly outdated.

  A good starting point is of course the Metalab LDP archive
  <http://metalab.unc.edu/LDP/> that is a information central for
  documentation, project pages and much, much more.



  ╖  Mike Neuffer, the author of the DPT caching RAID controller
     drivers, has some interesting pages on SCSI <http://www.uni-
     mainz.de/~neuffer/scsi> and DPT <http://www.uni-
     mainz.de/~neuffer/scsi/dpt>.

  ╖  Software RAID development information can be found at Linux Kernel
     site <http://www.kernel.org/> along with patches and utilities.

  ╖  Disk related information on benchmarking, RAID, reliability and
     much, much more can be found at Linas Vepstas <http://linas.org>
     project page.

  ╖  There is also information available on how to RAID the root
     partition <ftp://ftp.bizsystems.com/pub/raid/Root-RAID-HOWTO.html>
     and what software packages are needed to achieve this.
  ╖  In depth documentation on ext2fs
     <http://step.polymtl.ca/~ldd/ext2fs/ext2fs_toc.html> is also
     available.



  ╖  People who looking for information on VFAT, FAT32 and Joliet could
     have a look at the development page
     <http://bmrc.berkeley.edu/people/chaffee/index.html>.  These
     drivers are now in the 2.1.x kernel development series as well as
     in 2.0.34 and later.


  ╖  For more information on booting and also some BSD information have
     a look at booting information
     <http://www.paranoia.com/~vax/boot.html> page.

  For diagrams and information on all sorts of disk drives, controllers
  etc. both for current and discontinued lines The Ref
  <http://theref.c3d.rl.af.mil> is the site you need. There is a lot of
  useful information here, a real treasure trove.  You can also download
  the database using FTP <ftp://theref.c3d.rl.af.mil/public>.

  Please let me know if you have any other leads that can be of
  interest.




  15.7.  Search Engines

  Remember you can also use the web search engines and that some, like

  ╖  Altavista <http://www.altavista.digital.com>

  ╖  Excite <http://www.excite.com>

  ╖  Hotbot <http://www.hotbot.com>

     can also search usenet news.

  Also remember that Dejanews <http://www.dejanews.com> is a dedicated
  news searcher that keeps a news spool from early 1995 and onwards.


  If you have to ask for help you are most likely to get help in the
  Linux Setup <news:comp.os.linux.setup> news group.  Due to large
  workload and a slow network connection I am not able to follow that
  newsgroup so if you want to contact me you have to do so by e-mail.



  16.  Getting Help


  In the end you might find yourself unable to solve your problems and
  need help from someone else. The most efficient way is either to ask
  someone local or in your nearest Linux user group, search the web for
  the nearest one.

  Another possibility is to ask on Usenet News in one of the many, many
  newsgroups available. The problem is that these have such a high
  volume and noise (called low signal-to-noise ratio) that your question
  can easily fall through unanswered.


  No matter where you ask it is important to ask well or you will not be
  taken seriously. Saying just my disk does not work is not going to
  help you and instead the noise level is increased even further and if
  you are lucky someone will ask you to clarify.

  Instead describe your problems in some detail that will enable people
  to help you. The problem could lie somewhere you did not expect.
  Therefore you are advised to list up the following information on your
  system:


     Hardware

     ╖  Processor

     ╖  DMA

     ╖  IRQ

     ╖  Chip set (LX, BX etc)

     ╖  Bus (ISA, VESA, PCI etc)

     ╖  Expansion cards used (Disk controllers, video, IO etc)


     Software

     ╖  BIOS (On motherboard and possibly SCSI host adapters)

     ╖  LILO, if used

     ╖  Linux kernel version as well as possible modifications and
        patches

     ╖  Kernel parameters, if any

     ╖  Software that shows the error (with version number or date)


     Peripherals

     ╖  Type of disk drives with manufacturer name, version and type

     ╖  Other relevant peripherals connected to the same busses


  As an example of how interrelated these problems are: an old chip set
  caused problems with a certain combination of video controller and
  SCSI host adapter.

  Remember that booting text is logged to /var/log/messages which can
  answer most of the questions above. Obviously if the drives fail you
  might not be able to get  the log saved to disk but you can at least
  scroll back up the screen using the SHIFT and PAGE UP keys. It may
  also be useful to include part of this in your request for help but do
  not go overboard, keep it brief as a complete log file dumped to
  Usenet News is more than a little annoying.



  17.  Concluding Remarks

  Disk tuning and partition decisions are difficult to make, and there
  are no hard rules here. Nevertheless it is a good idea to work more on
  this as the payoffs can be considerable. Maximizing usage on one drive
  only while the others are idle is unlikely to be optimal, watch the
  drive light, they are not there just for decoration. For a properly
  set up system the lights should look like Christmas in a disco. Linux
  offers software RAID but also support for some hardware base SCSI RAID
  controllers. Check what is available. As your system and experiences
  evolve you are likely to repartition and you might look on this
  document again. Additions are always welcome.

  Finally I'd like to sum up my recommendations:

  ╖  Disk is cheap but the data they contain could be much more
     valuable, use and test your backup system.

  ╖  Work is also expensive, make sure you get large enough disks as
     refitting new or repartitioning old disks takes time.

  ╖  Think reliability, replace old disks before they fail.

  ╖  Keep a paper copy of your setup, having it all on disk when the
     machine is down will not help you much.

  ╖  Start out with a simple design with a minimum of fancy technology
     and rather fit it in later. In general adding is easier than
     replacing, be it disks, technology or other features.



  17.1.  Coming Soon

  There are a few more important things that are about to appear here.
  In particular I will add more example tables as I am about to set up
  two fairly large and general systems, one at work and one at home.
  These should give some general feeling on how a system can be set up
  for either of these two purposes. Examples of smooth running existing
  systems are also welcome.

  There is also a fair bit of work left to do on the various kinds of
  file systems and utilities.

  There will be a big addition on drive technologies coming soon as well
  as a more in depth description on using fdisk, cfdisk and sfdisk.  The
  file systems will be beefed up as more features become available as
  well as more on RAID and what directories can benefit from what RAID
  level.


  There is some minor overlapping with the Linux Filesystem Structure
  Standard and FHS that I hope to integrate better soon, which will
  probably mean a big reworking of all the tables at the end of this
  document.

  As more people start reading this I should get some more comments and
  feedback. I am also thinking of making a program that can automate a
  fair bit of this decision making process and although it is unlikely
  to be optimum it should provide a simpler, more complete starting
  point.


  17.2.  Request for Information

  It has taken a fair bit of time to write this document and although
  most pieces are beginning to come together there are still some
  information needed before we are out of the beta stage.



  ╖  More information on swap sizing policies is needed as well as
     information on the largest swap size possible under the various
     kernel versions.

  ╖  How common is drive or file system corruption? So far I have only
     heard of problems caused by flaky hardware.

  ╖  References to speed and drives is needed.

  ╖  Are any other Linux compatible RAID controllers available?

  ╖  What relevant monitoring, management and maintenance tools are
     available?

  ╖  General references to information sources are needed, perhaps this
     should be a separate document?

  ╖  Usage of /tmp and /var/tmp has been hard to determine, in fact what
     programs use which directory is not well defined and more
     information here is required. Still, it seems at least clear that
     these should reside on different physical drives in order to
     increase paralellicity.


  17.3.  Suggested Project Work

  Now and then people post on comp.os.linux.*, looking for good project
  ideas. Here I will list a few that comes to mind that are relevant to
  this document. Plans about big projects such as new file systems
  should still be posted in order to either find co-workers or see if
  someone is already working on it.



     Planning tools
        that can automate the design process outlines earlier would
        probably make a medium sized project, perhaps as an exercise in
        constraint based programming.


     Partitioning tools
        that take the output of the previously mentioned program and
        format drives in parallel and apply the appropriate symbolic
        links to the directory structure. It would probably be best if
        this were integrated in existing system installation software.
        The drive partitioning setup used in Solaris is an example of
        what it can look like.


     Surveillance tools
        that keep an eye on the partition sizes and warn before a
        partition overflows.


     Migration tools
        that safely lets you move old structures to new (for instance
        RAID) systems. This could probably be done as a shell script
        controlling a back up program and would be rather simple. Still,
        be sure it is safe and that the changes can be undone.



  18.  Questions and Answers

  This is just a collection of what I believe are the most common
  questions people might have. Give me more feedback and I will turn
  this section into a proper FAQ.


  ╖  Q:How many physical disk drives (spindles) does a Linux system
     need?

     A: Linux can run just fine on one drive (spindle).  Having enough
     RAM (around 32 MB, and up to 64 MB) to support swapping is a better
     price/performance choice than getting a second disk.  (E)IDE disk
     is usually cheaper (but a little slower) than SCSI.


  ╖  Q: I have a single drive, will this HOWTO help me?

     A: Yes, although only to a minor degree. Still, section ``Physical
     Track Positioning'' will offer you some gains.


  ╖  Q: Are there any disadvantages in this scheme?

     A: There is only a minor snag: if even a single partition overflows
     the system might stop working properly. The severity depends of
     course on what partition is affected. Still this is not hard to
     monitor, the command df gives you a good overview of the situation.
     Also check the swap partition(s) using free to make sure you are
     not about to run out of virtual memory.


  ╖  Q: OK, so should I split the system into as many partitions as
     possible for a single drive?

     A: No, there are several disadvantages to that. First of all
     maintenance becomes needlessly complex and you gain very little in
     this. In fact if your partitions are too big you will seek across
     larger areas than needed.  This is a balance and dependent on the
     number of physical drives you have.


  ╖  Q: Does that mean more drives allows more partitions?

     A: To some degree, yes. Still, some directories should not be split
     off from root, check out the file system standards for more
     details.


  ╖  Q: What if I have many drives I want to use?

     A: If you have more than 3-4 drives you should consider using RAID
     of some form. Still, it is a good idea to keep your root partition
     on a simple partition without RAID, see section ``RAID'' for more
     details.


  ╖  Q: I have installed the latest Windows95 but cannot access this
     partition from within the Linux system, what is wrong?

     A: Most likely you are using FAT32 in your windows partition. It
     seems that Microsoft decided we needed yet another format, and this
     was introduced in their latest version of Windows95, called OSR2.
     The advantage is that this format is better suited to large drives.

     You might also be interested to hear that Microsoft NT 4.0 does not
     support it yet either.



  ╖  Q: I cannot get the disk size and partition sizes to match,
     something is missing. What has happened?

     A:It is possible you have mounted a partition onto a mount point
     that was not an empty directory. Mount points are directories and
     if it is not empty the mounting will mask the contents. If you do
     the sums you will see the amount of disk space used in this
     directory is missing from the observed total.

     To solve this you can boot from a rescue disk and see what is
     hiding behind your mount points and remove or transfer the contents
     by mounting the offending partition on a temporary mounting point.
     You might find it useful to have "spare" emergency mounting points
     ready made.


  ╖  Q: It doesn't look like my swap partition is in use, how come?

     A: It is possible that it has not been necessary to swap out,
     especially if you have plenty of RAM. Check your log files to see
     if you ran out of memory at one point or another, in that case your
     swap space should have been put to use. If not it is possible that
     either the swap partition was not assigned the right number, that
     you did not prepare it with mkswap or that you have not done swapon
     or added it to your fstab.



  ╖  Q: What is this nyx that is mentioned several times here?

     A: It is a large free Unix system with currently about 10000 users.
     I use it for my web pages for this HOWTO as well as a source of
     ideas for a setup of large Unix systems. It has been running for
     many years and has a quite stable setup. For more information you
     can view the Nyx homepage <http://www.nyx.net> which also gives you
     information on how to get your own free account.




  19.  Bits and Pieces

  This is basically a section where I stuff all the bits I have not yet
  decided where should go, yet that I feel is worth knowing about. It is
  a kind of transient area.




  19.1.  Swap Partition: to Use or Not to Use

  In many cases you do not need a swap partition, for instance if you
  have plenty of RAM, say, more than 64 MB, and you are the sole user of
  the machine. In this case you can experiment running without a swap
  partition and check the system logs to see if you ran out of virtual
  memory at any point.

  Removing swap partitions have two advantages:

  ╖  you save disk space (rather obvious really)

  ╖  you save seek time as swap partitions otherwise would lie in the
     middle of your disk space.

  In the end, having a swap partition is like having a heated toilet:
  you do not use it very often, but you sure appreciate it when you
  require it.


  19.2.  Mount Point and /mnt

  In an earlier version of this document I proposed to put all
  permanently mounted partitions under /mnt. That, however, is not such
  a good idea as this itself can be used as a mount point, which leads
  to all mounted partitions becoming unavailable. Instead I will propose
  mounting straight from root using a meaningful name like
  /mnt.descriptive-name.

  Lately I have become aware that some Linux distributions use mount
  points at subdirectories under /mnt, such as /mnt/floppy and
  /mnt/cdrom, which just shows how confused the whole issue is.
  Hopefully FHS should clarify this.



  19.3.  Power and Heating

  Not many years ago a machine with the equivalent power of a modern PC
  required 3-phase power and cooling, usually by air conditioning the
  machine room, some times also by water cooling. Technology has
  progressed very quickly giving not only high speed but also low power
  components. Still, there is a definite limit to the technology,
  something one should keep in mind as the system is expanded with yet
  another disk drive or PCI card. When the power supply is running at
  full rated power, keep in mind that all this energy is going
  somewhere, mostly into heat. Unless this is dissipated using fans you
  will get a serious heating inside the cabinet followed by a reduced
  reliability and also life time of the electronics.  Manufacturers
  state minimum cooling requirements for their drives, usually in terms
  of cubic feet per minute (CFM). You are well advised to take this
  serious.

  Keep air flow passages open, clean out dust and check the temperature
  of your system running. If it is too hot to touch it is probably
  running too hot.

  If possible use sequential spin up for the drives. It is during spin
  up, when the drive platters accelerate up to normal speed, that a
  drive consumes maximum power and if all drives start up simultaneously
  you could go beyond the rated power maximum of your power supply.


  19.4.  Dejanews

  This is an Internet system that no doubt most of you are familiar
  with.  It searches and serves Usenet News articles from 1995 and to
  the latest postings and also offers a web based reading and posting
  interface.  There is a lot more, check out Dejanews
  <http://www.dejanews.com> for more information.

  What perhaps is less known, is that they use about 120 Linux SMP
  computers many of which use the md module to manage between 4 and 24
  Gig of disk space (over 1200 Gig altogether) for this service.  The
  system is continuously growing but at the time of writing they use
  mostly dual Pentium Pro 200MHz and Pentium II 300 MHz systems with 256
  MB RAM or more.

  A production database machine normally has 1 disk for the operating
  system and between 4 and 6 disks managed by the md module where the
  articles are archived.  The drives are connected to BusLogic Model
  BT-946C and BT-958 PCI SCSI adapters, usually one to a machine.

  For the production systems (which are up 365 days a year) the downtime
  attributable to disk errors is less than 0.25 % (that is a quarter of
  1%, not 25%).

  Just in case: this is not an advertisement, it is stated as an example
  of how much is required for what is a major Internet service.






  20.  Appendix A: Partitioning Layout Table: Mounting and Linking

  The following table is designed to make layout a simpler paper and
  pencil exercise. It is probably best to print it out (using NON
  PROPORTIONAL fonts) and adjust the numbers until you are happy with
  them.

  Mount point is what directory you wish to mount a partition on or the
  actual device. This is also a good place to note how you plan to use
  symbolic links.

  The size given corresponds to a fairly big Debian 1.2.6 installation.
  Other examples are coming later.

  Mainly you use this table to select what structure and drives you will
  use, the partition numbers and letters will come from the next two
  tables.





































  Directory       Mount point     speed   seek    transfer        size    SIZE


  swap            __________      ooooo   ooooo   ooooo           32      ____

  /               __________      o       o       o               20      ____

  /tmp            __________      oooo    oooo    oooo                    ____

  /var            __________      oo      oo      oo              25      ____
  /var/tmp        __________      oooo    oooo    oooo                    ____
  /var/spool      __________                                              ____
  /var/spool/mail __________      o       o       o                       ____
  /var/spool/news __________      ooo     ooo     oo                      ____
  /var/spool/____ __________      ____    ____    ____                    ____

  /home           __________      oo      oo      oo                      ____

  /usr            __________                                      500     ____
  /usr/bin        __________      o       oo      o               250     ____
  /usr/lib        __________      oo      oo      ooo             200     ____
  /usr/local      __________                                              ____
  /usr/local/bin  __________      o       oo      o                       ____
  /usr/local/lib  __________      oo      oo      ooo                     ____
  /usr/local/____ __________                                              ____
  /usr/src        __________      o       oo      o               50      ____

  DOS             __________      o       o       o                       ____
  Win             __________      oo      oo      oo                      ____
  NT              __________      ooo     ooo     ooo                     ____

  /mnt._________  __________      ____    ____    ____                    ____
  /mnt._________  __________      ____    ____    ____                    ____
  /mnt._________  __________      ____    ____    ____                    ____
  /_____________  __________      ____    ____    ____                    ____
  /_____________  __________      ____    ____    ____                    ____
  /_____________  __________      ____    ____    ____                    ____



  Total capacity:






  21.  Appendix B: Partitioning Layout Table: Numbering and Sizing

  This table follows the same logical structure as the table above where
  you decided what disk to use. Here you select the physical tracking,
  keeping in mind the effect of track positioning mentioned earlier in
  ``Physical Track Positioning''.

  The final partition number will come out of the table after this.











    Drive           sda     sdb     sdc     hda     hdb     hdc     ___

  SCSI ID         |  __   |  __   |  __   |

  Directory
  swap            |       |       |       |       |       |       |

  /               |       |       |       |       |       |       |

  /tmp            |       |       |       |       |       |       |

  /var            :       :       :       :       :       :       :
  /var/tmp        |       |       |       |       |       |       |
  /var/spool      :       :       :       :       :       :       :
  /var/spool/mail |       |       |       |       |       |       |
  /var/spool/news :       :       :       :       :       :       :
  /var/spool/____ |       |       |       |       |       |       |

  /home           |       |       |       |       |       |       |

  /usr            |       |       |       |       |       |       |
  /usr/bin        :       :       :       :       :       :       :
  /usr/lib        |       |       |       |       |       |       |
  /usr/local      :       :       :       :       :       :       :
  /usr/local/bin  |       |       |       |       |       |       |
  /usr/local/lib  :       :       :       :       :       :       :
  /usr/local/____ |       |       |       |       |       |       |
  /usr/src        :       :       :       :

  DOS             |       |       |       |       |       |       |
  Win             :       :       :       :       :       :       :
  NT              |       |       |       |       |       |       |

  /mnt.___/_____  |       |       |       |       |       |       |
  /mnt.___/_____  :       :       :       :       :       :       :
  /mnt.___/_____  |       |       |       |       |       |       |
  /_____________  :       :       :       :       :       :       :
  /_____________  |       |       |       |       |       |       |
  /_____________  :       :       :       :       :       :       :


  Total capacity:






  22.  Appendix C: Partitioning Layout Table: Partition Placement

  This is just to sort the partition numbers in ascending order ready to
  input to fdisk or cfdisk. Here you take physical track positioning
  into account when finalizing your design. Unless you get specific
  information otherwise, you can assume track 0 is the outermost track.

  These numbers and letters are then used to update the previous tables,
  all of which you will find very useful in later maintenance.

  In case of disk crash you might find it handy to know what SCSI id
  belongs to which drive, consider keeping a paper copy of this.






          Drive :   sda     sdb     sdc     hda     hdb     hdc     ___

  Total capacity: |  ___  |  ___  |  ___  |  ___  |  ___  |  ___  |  ___
  SCSI ID         |  __   |  __   |  __   |

  Partition

  1               |       |       |       |       |       |       |
  2               :       :       :       :       :       :       :
  3               |       |       |       |       |       |       |
  4               :       :       :       :       :       :       :
  5               |       |       |       |       |       |       |
  6               :       :       :       :       :       :       :
  7               |       |       |       |       |       |       |
  8               :       :       :       :       :       :       :
  9               |       |       |       |       |       |       |
  10              :       :       :       :       :       :       :
  11              |       |       |       |       |       |       |
  12              :       :       :       :       :       :       :
  13              |       |       |       |       |       |       |
  14              :       :       :       :       :       :       :
  15              |       |       |       |       |       |       |
  16              :       :       :       :       :       :       :






  23.  Appendix D: Example: Multipurpose Server

  The following table is from the setup of a medium sized multipurpose
  server where I work. Aside from being a general Linux machine it will
  also be a network related server (DNS, mail, FTP, news, printers etc.)
  X server for various CAD programs, CD ROM burner and many other
  things.  The files reside on 3 SCSI drives with a capacity of 600,
  1000 and 1300 MB.

  Some further speed could possibly be gained by splitting /usr/local
  from the rest of the /usr system but we deemed the further added
  complexity would not be worth it. With another couple of drives this
  could be more worthwhile. In this setup drive sda is old and slow and
  could just a well be replaced by an IDE drive. The other two drives
  are both rather fast. Basically we split most of the load between
  these two. To reduce dangers of imbalance in partition sizing we have
  decided to keep /usr/bin and /usr/local/bin in one drive and /usr/lib
  and /usr/local/lib on another separate drive which also affords us
  some drive parallelizing.

  Even more could be gained by using RAID but we felt that as a server
  we needed more reliability than was then afforded by the md patch and
  a dedicated RAID controller was out of our reach.


  24.  Appendix E: Example: Mounting and Linking











  Directory       Mount point     speed   seek    transfer        size    SIZE


  swap            sdb2, sdc2      ooooo   ooooo   ooooo           32      2x64

  /               sda2            o       o       o               20       100

  /tmp            sdb3            oooo    oooo    oooo                     300

  /var            __________      oo      oo      oo                      ____
  /var/tmp        sdc3            oooo    oooo    oooo                     300
  /var/spool      sdb1                                                     436
  /var/spool/mail __________      o       o       o                       ____
  /var/spool/news __________      ooo     ooo     oo                      ____
  /var/spool/____ __________      ____    ____    ____                    ____

  /home           sda3            oo      oo      oo                       400

  /usr            sdb4                                            230      200
  /usr/bin        __________      o       oo      o               30      ____
  /usr/lib        -> libdisk      oo      oo      ooo             70      ____
  /usr/local      __________                                              ____
  /usr/local/bin  __________      o       oo      o                       ____
  /usr/local/lib  -> libdisk      oo      oo      ooo                     ____
  /usr/local/____ __________                                              ____
  /usr/src        ->/home/usr.src o       oo      o               10      ____

  DOS             sda1            o       o       o                        100
  Win             __________      oo      oo      oo                      ____
  NT              __________      ooo     ooo     ooo                     ____

  /mnt.libdisk    sdc4            oo      oo      ooo                      226
  /mnt.cd         sdc1            o       o       oo                       710


  Total capacity: 2900 MB






  25.  Appendix F: Example: Numbering and Sizing

  Here we do the adjustment of sizes and positioning.





















  Directory         sda     sdb     sdc


  swap            |       |   64  |   64  |

  /               |  100  |       |       |

  /tmp            |       |  300  |       |

  /var            :       :       :       :
  /var/tmp        |       |       |  300  |
  /var/spool      :       :  436  :       :
  /var/spool/mail |       |       |       |
  /var/spool/news :       :       :       :
  /var/spool/____ |       |       |       |

  /home           |  400  |       |       |

  /usr            |       |  200  |       |
  /usr/bin        :       :       :       :
  /usr/lib        |       |       |       |
  /usr/local      :       :       :       :
  /usr/local/bin  |       |       |       |
  /usr/local/lib  :       :       :       :
  /usr/local/____ |       |       |       |
  /usr/src        :       :       :       :

  DOS             |  100  |       |       |
  Win             :       :       :       :
  NT              |       |       |       |

  /mnt.libdisk    |       |       |  226  |
  /mnt.cd         :       :       :  710  :
  /mnt.___/_____  |       |       |       |


  Total capacity: |  600  | 1000  | 1300  |






  26.  Appendix G: Example: Partition Placement

  This is just to sort the partition numbers in ascending order ready to
  input to fdisk or cfdisk. Remember to optimize for physical track
  positioning (not done here).



               Drive :   sda     sdb     sdc

       Total capacity: |   600 |  1000 |  1300 |

       Partition

       1               |   100 |   436 |   710 |
       2               :   100 :    64 :    64 :
       3               |   400 |   300 |   300 |
       4               :       :   200 :   226 :





  27.  Appendix H: Example II


  The following is an example of a server setup in an academic setting,
  and is contributed by nakano (at) apm.seikei.ac.jp. I have only done
  minor editing to this section.

  /var/spool/delegate is a directory for storing logs and cache files of
  an WWW proxy server program, "delegated". Since I don't notice it
  widely, there are 1000--1500 requests/day currently, and average disk
  usage is 15--30% with expiration of caches each day.

  /mnt.archive is used for data files which are big and not frequently
  referenced such a s experimental data (especially graphic ones),
  various source archives, and Win95 backups (growing very fast...).

  /mnt.root is backup root file system containing rescue utilities. A
  boot floppy is also prepared to boot with this partition.
















































  =================================================
  Directory               sda      sdb     hda

  swap                    |    64 |    64 |       |
  /                       |       |       |    20 |
  /tmp                    |       |       |   180 |

  /var                    :   300 :       :       :
  /var/tmp                |       |   300 |       |
  /var/spool/delegate     |   300 |       |       |

  /home                   |       |       |   850 |
  /usr                    |   360 |       |       |
  /usr/lib                -> /mnt.lib/usr.lib
  /usr/local/lib          -> /mnt.lib/usr.local.lib

  /mnt.lib                |       |   350 |       |
  /mnt.archive            :       :  1300 :       :
  /mnt.root               |       |    20 |       |

  Total capacity:            1024    2034    1050


  =================================================
          Drive :           sda     sdb     hda
  Total capacity:         |  1024 |  2034 |  1050 |

  Partition
  1                       |   300 |    20 |    20 |
  2                       :    64 :  1300 :   180 :
  3                       |   300 |    64 |   850 |
  4                       :   360 :   ext :       :
  5                       |       |   300 |       |
  6                       :       :   350 :       :


  Filesystem         1024-blocks  Used Available Capacity Mounted on
  /dev/hda1              19485   10534     7945     57%   /
  /dev/hda2             178598      13   169362      0%   /tmp
  /dev/hda3             826640  440814   343138     56%   /home
  /dev/sda1             306088   33580   256700     12%   /var
  /dev/sda3             297925   47730   234807     17%   /var/spool/delegate
  /dev/sda4             363272  170872   173640     50%   /usr
  /dev/sdb5             297598       2   282228      0%   /var/tmp
  /dev/sdb2            1339248  302564   967520     24%   /mnt.archive
  /dev/sdb6             323716   78792   228208     26%   /mnt.lib




  Apparently /tmp and /var/tmp is too big. These directories shall be
  packed together into one partition when disk space shortage comes.

  /mnt.lib is also seemed to be, but I plan to install newer TeX and
  ghostscript archives, so /usr/local/lib may grow about 100 MB or so
  (since we must use Japanese fonts!).

  Whole system is backed up by Seagate Tapestore 8000 (Travan TR-4,
  4G/8G).







  28.  Appendix I: Example III: SPARC Solaris


  The following section is the basic design used at work for a number of
  Sun SPARC servers running Solaris 2.5.1 in an industrial development
  environment. It serves a number of database and cad applications in
  addition to the normal services such as mail.

  Simplicity is emphasized here so /usr/lib has not been split off from
  /usr.

  This is the basic layout, planned for about 100 users.



          Drive:        SCSI 0                      SCSI 1

          Partition     Size (MB)   Mount point    Size (MB)   Mount point

            0           160         swap           160         swap
            1           100         /tmp           100         /var/tmp
            2           400         /usr
            3           100         /
            4            50         /var
            5
            6           remainder   /local0        remainder   /local1




  Due to specific requirements at this place it is at times necessary to
  have large partitions available on a short notice. Therefore drive 0
  is given as many tasks as feasible, leaving a large /local1 partition.

  This setup has been in use for some time now and found satisfactorily.

  For a more general and balanced system it would be better to swap /tmp
  and /var/tmp and then move /var to drive 1.


  29.  Appendix J: Example IV: Server with 4 Drives

  This gives an example of using all techniques described earlier, short
  of RAID. It is admittedly rather complicated but offers in return high
  performance from modest hardware. Dimensioning are skipped but
  reasonable figures can be found in previous examples.



       Partition       sda             sdb             sdc             sdd
                       ----            ----            ----            ----
               1       root            overview        lib             news
               2       swap            swap            swap            swap
               3       home            /usr            /var/tmp        /tmp
               4                       spare root      mail            /var




  Setup is optimised with respect to track positioning but also for
  minimising drive seeks.

  If you want DOS or Windows too you will have to use sda1 for this and
  move the other partitions after that. It will be advantageous to use
  the swap partitions on sdb2, sdc2 and sdd2 for Windows swap, TEMPDIR
  and Windows temporary directory under these sessions. A number of
  other HOWTOs describe how you can make several operating systems
  coexist on your machine.


  For completeness a 4 drive example using several types of RAID is also
  given which is even more complex than the example above.



       Partition       sda             sdb             sdc             sdd
                       ----            ----            ----            ----
               1       boot            overview        news            news
               2       overview        swap            swap            swap
               3       swap            lib             lib             lib
               4       lib             overview        /tmp            /tmp
               5       /var/tmp        /var/tmp        mail            /usr
               6       /home           /usr            /usr            mail
               7       /usr            /home           /var
               8       / (root)        spare root




  Here all duplicates are parts of a RAID 0 set with two exceptions,
  swap which is interleaved and home and mail which are implemented as
  RAID 1 for safety.

  Note that boot and root are separated: only the boot file with the
  kernel has to reside within the 1023 cylinder limit. The rest of the
  root files can be anywhere and here they are placed on the slowest
  outermost partition. For simplicity and safety the root partition is
  not on a RAID system.

  With such a complicated comes an equally complicated fstab file.  The
  large number of partitions makes it important to do the fsck passes in
  the right order, otherwise the process can take perhaps ten times as
  long time to complete as the optimal solution.




       /dev/sda8       /               ?       ?               1 1 (a)
       /dev/sdb8       /               ?       noauto          1 2 (b)
       /dev/sda1       boot            ?       ?               1 2 (a)
       /dev/sdc7       /var            ?       ?               1 2 (c)
       /dev/md1        news            ?       ?               1 3 (c+d)
       /dev/md2        /var/tmp        ?       ?               1 3 (a+b)
       /dev/md3        mail            ?       ?               1 4 (c+d)
       /dev/md4        /home           ?       ?               1 4 (a+b)
       /dev/md5        /tmp            ?       ?               1 5 (c+d)
       /dev/md6        /usr            ?       ?               1 6 (a+b+c+d)
       /dev/md7        /lib            ?       ?               1 7 (a+b+c+d)




  The letters in the brackets indicate what drives will be active for
  each fsck entry and pass. These letters are not present in a real
  fstab file.  All in all there are 7 passes.



  30.  Appendix K: Example V: Dual Drive System

  A dual drive system offers less opportunity for clever schemes but the
  following should provide a simple starting point.
       Partition       sda             sdb
                       ----            ----
               1       boot            lib
               2       swap            news
               3       /tmp            swap
               4       /usr            /var/tmp
               5       /var            /home
               6       / (root)





  If you use a dual OS system you have to keep in mind that many other
  systems must boot from the first partition on the first drive. A
  simple DOS / Linux system could look like this:



       Partition       sda             sdb
                       ----            ----
               1       DOS             lib
               2       boot            news
               3       swap            swap
               4       /tmp            /var/tmp
               5       /usr            /home
               6       /var            DOSTEMP
               7       / (root)





  Also remember that DOS and Windows prefer there to be just a single
  primary partition which has to be the first one where it boots from.
  As Linux can happily exist in logical partitions this is not a big
  problem.



  31.  Appendix L: Example VI: Single Drive System

  Although this falls somewhat outside the scope of this HOWTO it cannot
  be denied that recently some rather large drives have become very
  affordable. Drives with 10 - 20 GB are becoming common and the
  question often is how best to partition such monsters. Interestingly
  enough very few seem to have any problems in filling up such drives
  and the future looks generally quite rosy for manufacturers planning
  on even bigger drives.

  Opportunities for optimisations are of course even smaller than for 2
  drive systems but some tricks can still be used to optimise track
  positions while minimising head movements.













  Partition       hda             Size estimate (MB)
                  ----            ------------------
           1      DOS             500
           2      boot            20
           3      Winswap         200
           4      data            The bulk of the drive
           5      lib             50 - 500
           6      news            300+
           7      swap            128     (Maximum size for 32-bit CPU)
           8      tmp             300+    (/tmp and /var/tmp)
           9      /usr            50 - 500
          10      /home           300+
          11      /var            50 - 300
          12      mail            300+
          13      dosdata         10      ( Windows bug workaround!)




  Remember that the dosdata partition is a DOS filesystem that must be
  the very last partition on the drive, otherwise Windows gets confused.