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Linux Partition HOWTO Kristan Koehntopp, kris@koehntopp.de $Id: howto.sgml,v 2.4 1997/11/03 06:27:22 kris Exp $ This Linux Mini-HOWTO teaches you how to plan and layout disk space for your Linux system. It talks about disk hardware, partitions, swap space sizing and positioning considerations. file systems, file system types and related topics. The intent is to teach some background knowledge, not procedures. 1. Introduction 1.1. What is this? This is a Linux Mini-HOWTO text. A Mini-HOWTO is a small text explaining some business related to Linux installation and maintenance tutorial style. It's mini, because either the text or the topic it discusses are too small for a real HOWTO or even a book. A HOWTO is not a reference: that's what manual pages are for. 1.2. What is in it? and related HOWTO documents This particular Mini-HOWTO teaches you how to plan and layout disk space for your Linux system. It talks about disk hardware, partitions, swap space sizing and positioning considerations, file systems, file system types and related topics. The intent is to teach some background knowlegde, so we are talking mainly principles and not tools in this text. Ideally, this document should be read before your first installation, but this is somehow difficult for most people. First timers have other problems than disk layout optimization, too. So you are probably someone who just finished a Linux installation and is now thinking about ways to optimize this installation or how to avoid some nasty miscalculations in the next one. Well, expect some desire to tear down and rebuild your installation when you are finished with this text. :-) This Mini-HOWTO limits itself to planning and layouting disk space most of the time. It does not discuss the usage of fdisk, LILO, mke2fs or backup programs. There are other HOWTOs that address these problems. Please see the Linux HOWTO Index for current information on Linux HOWTOs. There are instructions for obtaining HOWTO documents in the index, too. To learn how to estimate the various size and speed requirements for different parts of the filesystem, see "Linux Multiple Disks Layout mini-HOWTO", by Gjoen Stein <gjoen@nyx.net>. For instructions and considerations regarding disks with more than 1024 cylinders, see "Linux Large Disk mini-HOWTO", Andries Brouwer <aeb@cwi.nl>. For instructions on limiting disk space usage per user (quotas), see "Linux Quota mini-HOWTO", by Albert M.C. Tam <bertie@scn.org> Currently, there is no general document on disk backup, but there are several documents with pointers to specific backup solutions. See "Linux ADSM Backup mini-HOWTO", by Thomas Koenig <Thomas.Koenig@ciw.uni-karlsruhe.de> for instructions on integrating Linux into an IBM ADSM backup environment. See "Linux Backup with MSDOS mini-HOWTO", by Christopher Neufeld <neufeld@physics.utoronto.ca> for information about MS-DOS driven Linux backups. For instructions on writing and submitting a HOWTO document, see the Linux HOWTO Index, by Greg Hankins <gregh@sunsite.unc.edu>. Browsing through /usr/src/linux/Documentation can be very instructive, too. See ide.txt and scsi.txt for some background information on the properties of your disk drivers and have a look at the filesystems/ subdirectory. 2. What is a partition anyway? When PC hard disks were invented people soon wanted to install multiple operating systems, even if their system had only one disk. So a mechanism was needed to divide a single physical disk into multiple logical disks. So that's what a partition is: A contiguous section of blocks on your hard disk that is treated like a completely seperate disk by most operating systems. It is fairly clear that partitions must not overlap: An operating system will certainly not be pleased, if another operating system installed on the same machine were overwriting important information because of overlapping partitions. There should be no gap between adjacent partitions, too. While this constellation is not harmful, you are wasting precious disk space by leaving space between partitions. A disk need not be partitioned completely. You may decide to leave some space at the end of your disk that is not assigned to any of your installed operating systems, yet. Later, when it is clear which installation is used by you most of the time, you can partition this left over space and put a file system on it. Partitions can not be moved nor can they be resized without destroying the file system contained in it. So repartitioning usually involves backup and restore of all file systems touched during the repartitioning. In fact it is fairly common to mess up things completely during repartitioning, so you should back up anything on any disk on that particular machine before even touching things like fdisk. Well, some partitions with certain file system types on them actually can be split into two without losing any data (if you are lucky). For example there is a program called "fips" for splitting MS-DOS partitions into two to make room for a Linux installation without having to reinstall MS-DOS. You are still not going to touch these things without carefully backing up everything on that machine, aren't you? 2.1. Backups are important Tapes are your friend for backups. They are fast, reliable and easy to use, so you can make backups often, preferably automatically and without hassle. Step on soapbox: And I am talking about real tapes, not that disk controller driven ftape crap. Consider buying SCSI: Linux does support SCSI natively. You don't need to load ASPI drivers, you are not losing precious HMA under Linux and once the SCSI host adapter is installed, you just attach additional disks, tapes and CD-ROMs to it. No more I/O addresses, IRQ juggling or Master/Slave and PIO-level matching. Plus: Proper SCSI host adapters give you high I/O performance without much CPU load. Even under heavy disk activity you will experience good response times. If you are planning to use a Linux system as a major USENET news feed or if you are about to enter the ISP business, don't even think about deploying a system without SCSI. Climb of soapbox. 2.2. Device numbers and device names The number of partitions on an Intel based system was limited from the very beginning: The original partition table was installed as part of the boot sector and held space for only four partition entries. These partitions are now called primary partitions. When it became clear that people needed more partitions on their systems, logical partitions were invented. The number of logical partitions is not limited: Each logical partition contains a pointer to the next logical partition, so you can have a potentially unlimited chain of partition entries. For compatibility reasons, the space occupied by all logical partitions had to be accounted for. If you are using logical partitions, one primary partition entry is marked as "extended partition" and its starting and ending block mark the area occupied by your logical partitions. This implies that the space assigned to all logical partitions has to be contiguous. There can be only one extended partition: no fdisk program will create more than one extended partition. Linux cannot handle more than a limited number of partitions per drive. So in Linux you have 4 primary partitions (3 of them useable, if you are using logical partitions) and at most 15 partitions altogether on an SCSI disk (63 altogether on an IDE disk). In Linux, partitions are represented by device files. A device file is a file with type c (for "character" devices, devices that do not use the buffer cache) or b (for "block" devices, which go through the buffer cache). In Linux, all disks are represented as block devices only. Unlike other Unices, Linux does not offer "raw" character versions of disks and their partitions. The only important thing with a device file are its major and minor device number, shown instead of the files size: ______________________________________________________________________ $ ls -l /dev/hda brw-rw---- 1 root disk 3, 0 Jul 18 1994 /dev/hda ^ ^ | minor device number major device number ______________________________________________________________________ When accessing a device file, the major number selects which device driver is being called to perform the input/output operation. This call is being done with the minor number as a parameter and it is entirely up to the driver how the minor number is being interpreted. The driver documentation usually describes how the driver uses minor numbers. For IDE disks, this documentation is in /usr/src/linux/Documentation/ide.txt. For SCSI disks, one would expect such documentation in /usr/src/linux/Documentation/scsi.txt, but it isn't there. One has to look at the driver source to be sure (/usr/src/linux/driver/scsi/sd.c:184-196). Fortunately, there is Peter Anvin's list of device numbers and names in /usr/src/linux/Documentation/devices.txt; see the entries for block devices, major 3, 22, 33, 34 for IDE and major 8 for SCSI disks. The major and minor numbers are a byte each and that is why the number of partitions per disk is limited. By convention device files have certain names and many system programs have knowledge about these names compiled in. They expect your IDE disks to be named /dev/hd* and your SCSI disks to be named /dev/sd*. Disks are numbered a, b, c and so on, so /dev/hda is your first IDE disk and /dev/sda is your first SCSI disk. Both devices represent entire disks, starting at block one. Writing to these devices with the wrong tools will destroy the master boot loader and partition table on these disks, rendering all data on this disk unusable or making your system unbootable. Know what you are doing and, again, back up before you do it. Primary partitions on a disk are 1, 2, 3 and 4. So /dev/hda1 is the first primary partition on the first IDE disk and so on. Logical partitions have numbers 5 and up, so /dev/sdb5 is the first logical partition on the second SCSI disk. Each partition entry has a starting and an ending block address assigned to it and a type. The type is a numerical code (a byte) which designates a particular partition to a certain type of operating system. For the benefit of computing consultants partition type codes are not really unique, so there is always the probability of two operating systems using the same type code. Linux reserves the type code 0x82 for swap partitions and 0x83 for "native" file systems (that's ext2 for almost all of you). The once popular, now outdated Linux/Minix file system used the type code 0x81 for partitions. OS/2 marks it's partitions with a 0x07 type and so does Windows NT's NTFS. MS-DOS allocates several type codes for its various flavors of FAT file systems: 0x01, 0x04 and 0x06 are known. DR-DOS used 0x81 to indicate protected FAT partitions, creating a type clash with Linux/Minix at that time, but neither Linux/Minix nor DR- DOS are widely used any more. The extended partition which is used as a container for logical partitions has a type of 0x05, by the way. Partitions are created and deleted with the fdisk program. Every self respecting operating system program comes with an fdisk and traditionally it is even called fdisk (or FDISK.EXE) in almost all OSes. Some fdisks, noteable the DOS one, are somehow limited when they have to deal with other operating systems partitions. Such limitations include the complete inability to deal with anything with a foreign type code, the inability to deal with cylinder numbers above 1024 and the inability to create or even understand partitions that do not end on a cylinder boundary. For example, the MS-DOS fdisk can't delete NTFS partitions, the OS/2 fdisk has been known to silently "correct" partitions created by the Linux fdisk that do not end on a cylinder boundary and both, the DOS and the OS/2 fdisk, have had problems with disks with more than 1024 cylinders (see the "large-disk" Mini-Howto for details on such disks). 3. What Partitions do I need? 3.1. How many partitions do I need? Okay, so what partitions do you need? Well, some operating systems do not believe in booting from logical partitions for reasons that are beyond the scope of any sane mind. So you probably want to reserve your primary partitions as boot partitions for your MS-DOS, OS/2 and Linux or whatever you are using. Remember that one primary partition is needed as an extended partition, which acts as a container for the rest of your disk with logical partitions. Booting operating systems is a real-mode thing involving BIOSes and 1024 cylinder limitations. So you probably want to put all your boot partitions into the first 1024 cylinders of your hard disk, just to avoid problems. Again, read the "large-disk" Mini-Howto for the gory details. To install Linux, you will need at least one partition. If the kernel is loaded from this partition (for example by LILO), this partition must be readable by your BIOS. If you are using other means to load your kernel (for example a boot disk or the LOADLIN.EXE MS-DOS based Linux loader) the partition can be anywhere. In any case this partition will be of type 0x83 "Linux native". Your system will need some swap space. Unless you swap to files you will need a dedicated swap partition. Since this partition is only accessed by the Linux kernel and the Linux kernel does not suffer from PC BIOS deficiencies, the swap partition may be positioned anywhere. I recommed using a logical partition for it (/dev/?d?5 and higher). Dedicated Linux swap partitions are of type 0x82 "Linux swap". These are minimal partition requirements. It may be useful to create more partitions for Linux. Read on. 3.2. How large should my swap space be? If you have decided to use a dedicated swap partition, which is generally a Good Idea tm, follow these guidelines for estimating its size: ╖ In Linux RAM and swap space add up (This is not true for all Unices). For example, if you have 8 MB of RAM and 12 MB swap space, you have a total of about 20 MB virtual memory. ╖ When sizing your swap space, you should have at least 16 MB of total virtual memory. So for 4 MB of RAM consider at least 12 MB of swap, for 8 MB of RAM consider at least 8 MB of swap. ╖ In Linux, a single swap partition can not be larger than 128 MB. That is, the partition may be larger than 128 MB, but excess space is never used. If you want more than 128 MB of swap, you have to create multiple swap partitions. ╖ When sizing swap space, keep in mind that too much swap space may not be useful at all. Every process has a "working set". This is a set of in-memory pages which will be referenced by the processor in the very near future. Linux tries to predict these memory accesses (assuming that recently used pages will be used again in the near future) and keeps these pages in RAM if possible. If the program has a good "locality of reference" this assumption will be true and prediction algorithm will work. Holding a working set in main memory does only work if there is enough main memory. If you have too many processes running on a machine, the kernel is forced to put pages on disk that it will reference again in the very near future (forcing a page-out of a page from another working set and then a page-in of the page referenced). Usually this results in a very heavy increase in paging activity and in a sustantial drop of performance. A machine in this state is said to be "thrashing" (For you german readers: That's "thrashing" ("dreschen", "schlagen", "haemmern") and not trashing ("muellen")). On a thrashing machine the processes are essentially running from disk and not from RAM. Expect performance to drop by approximately the ratio between memory access speed and disk access speed. A very old rule of thumb in the days of the PDP and the Vax was that the size of the working set of a program is about 25% of its virtual size. Thus it is probably useless to provide more swap than three times your RAM. But keep in mind that this is just a rule of thumb. It is easily possible to create scenarios where programs have extremely large or extremely small working sets. For example, a simulation program with a large data set that is accessed in a very random fashion would have almost no noticeable locality of reference in its data segment, so its working set would be quite large. On the other hand, an xv with many simultaneously opened JPEGs, all but one iconified, would have a very large data segment. But image transformations are all done on one single image, most of the memory occupied by xv is never touched. The same is true for an editor with many editor windows where only one window is being modified at a time. These programs have - if they are designed properly - a very high locality of reference and large parts of them can be kept swapped out without too severe performance impact. One could suspect that the 25% number from the age of the command line is no longer true for modern GUI programs editing multiple documents, but I know of no newer papers that try to verify these numbers. So for a configuration with 16 MB RAM, no swap is needed for a minimal configuration and more than 48 MB of swap are probably useless. The exact amount of memory needed depends on the application mix on the machine (what did you expect?). 3.3. Where should I put my swap space? ╖ Mechanics are slow, electronics are fast. Modern hard disks have many heads. Switching between heads of the same track is fast, since it is purely electronic. Switching between tracks is slow, since it involves moving real world matter. So if you have a disk with many heads and one with less heads and both are identical in other parameters, the disk with many heads will be faster. Splitting swap and putting it on both disks will be even faster, though. ╖ Older disks have the same number of sectors on all tracks. With this disks it will be fastest to put your swap in the middle of the disks, assuming that your disk head will move from a random track towards the swap area. ╖ Newer disks use ZBR (zone bit recording). They have more sectors on the outer tracks. With a constant number of rpms, this yields a far greater performance on the outer tracks than on the inner ones. Put your swap on the fast tracks. ╖ Of course your disk head will not move randomly. If you have swap space in the middle of a disk between a constantly busy home partition and an almost unused archive partition, you would be better of if your swap were in the middle of the home partition for even shorter head movements. You would be even better off, if you had your swap on another otherwise unused disk, though. Summary: Put your swap on a fast disk with many heads that is not busy doing other things. If you have multiple disks: Split swap and scatter it over all your disks or even different controllers. Even better: Buy more RAM. 3.4. Some facts about file systems and fragmentation Disk space is administered by the operating system in units of blocks and fragments of blocks. In ext2, fragments and blocks have to be of the same size, so we can limit our discussion to blocks. Files come in any size. They don't end on block boundaries. So with every file a part of the last block of every file is wasted. Assuming that file sizes are random, there is approximately a half block of waste for each file on your disk. Tanenbaum calls this "internal fragmentation" in his book "Operating Systems". You can guess the number of files on your disk by the number of allocated inodes on a disk. On my disk ______________________________________________________________________ # df -i Filesystem Inodes IUsed IFree %IUsed Mounted on /dev/hda3 64256 12234 52022 19% / /dev/hda5 96000 43058 52942 45% /var ______________________________________________________________________ there are about 12000 files on / and about 44000 files on /var. At a block size of 1 KB, about 6+22 = 28 MB of disk space are lost in the tail blocks of files. Had I chosen a block size of 4 KB, I had lost 4 times this space. Data transfer is faster for large contiguous chunks of data, though. That's why ext2 tries to preallocate space in units of 8 contigous blocks for growing files. Unused preallocation is released when the file is closed, so no space is wasted. Noncontiguous placement of blocks in a file is bad for performance, since files are often accessed in a sequential manner. It forces the operating system to split a disk access and the disk to move the head. This is called "external fragmentation" or simply "fragmentation" and is a common problem with DOS file systems. ext2 has several strategies to avoid external fragmentation. Normally fragmentation is not a large problem in ext2, not even on heavily used partitions such as a USENET news spool. While there is a tool for defragmentation of ext2 file systems, nobody ever uses it and it is not up to date with the current release of ext2. Use it, but do so on your own risk. The MS-DOS file system is well known for its pathological managment of disk space. In conjunction with the abysmal buffer cache used by MS- DOS the effects of file fragmentation on performance are very noticeable. DOS users are accustomed to defragging their disks every few weeks and some have even developed some ritualistic beliefs regarding defragmentation. None of these habits should be carried over to Linux and ext2. Linux native file systems do not need defragmentation under normal use and this includes any condition with at least 5% of free space on a disk. The MS-DOS file system is also known to lose large amounts of disk space due to internal fragmentation. For partitions larger than 256 MB, DOS block sizes grow so large that they are no longer useful (This has been corrected to some extent with FAT32). ext2 does not force you to choose large blocks for large file systems, except for very large file systems in the 0.5 TB range (that's terabytes with 1 TB equaling 1024 GB) and above, where small block sizes become inefficient. So unlike DOS there is no need to split up large disks into multiple partitions to keep block size down. Use the 1 KB default block size if possible. You may want to experiment with a block size of 2 KB for some partitions, but expect to meet some seldom exercised bugs: Most people use the default. 3.5. File lifetimes and backup cycles as partitioning criteria With ext2, Partitioning decisions should be governed by backup considerations and to avoid external fragmentation from different file lifetimes. Files have lifetimes. After a file has been created, it will remain some time on the system and then be removed. File lifetime varies greatly throughout the system and is partly dependent on the pathname of the file. For example, files in /bin, /sbin, /usr/sbin, /usr/bin and similar directories are likely to have a very long lifetime: many months and above. Files in /home are likely to have a medium lifetime: several weeks or so. File in /var are usually short lived: Almost no file in /var/spool/news will remain longer than a few days, files in /var/spool/lpd measure their lifetime in minutes or less. For backup it is useful if the amount of daily backup is smaller than the capacity of a single backup medium. A daily backup can be a complete backup or an incremental backup. You can decide to keep your partition sizes small enough that they fit completely onto one backup medium (choose daily full backups). In any case a partition should be small enough that its daily delta (all modified files) fits onto one backup medium (choose incremental backup and expect to change backup media for the weekly/monthly full dump - no unattended operation possible). Your backup strategy depends on that decision. When planning and buying disk space, remember to set aside a sufficient amount of money for backup! Unbackuped data is worthless! Data reproduction costs are much higher than backup costs for virtually everyone! For performance it is useful to keep files of different lifetimes on different partitions. This way the short lived files on the news partition may be fragmented very heavily. This has no impact on the performance of the / or /home partition. 4. An example 4.1. A recommended model for ambitious beginners A common model creates /, /home and /var partitions as discussed above. This is simple to install and maintain and differentiates well enough to avoid adverse effects from different lifetimes. It fits well into a backup model, too: Almost noone bothers to backup USENET news spools and only some files in /var are worth backing up (/var/spool/mail comes to mind). On the other hand, / changes infrequently and can be backuped upon demand (after configuration changes) and is small enough to fit on most modern backup media as a full backup (plan 250 to 500 MB depending on the amount of installed software). /home contains valuable user data and should be backuped daily. Some installations have very large /homes and must use incremental backups. Some systems put /tmp onto a seperate partition as well, others symlink it to /var/tmp to achieve the same effect (note that this can affect single user mode, where /var will be unavailable and the system will have no /tmp until you create one or mount /var manually) or put it onto a RAM disk (Solaris does this for example). This keeps /tmp out of /, a good idea. This model is convenient for upgrades or reinstallations as well: Save your configuration files (or the entire /etc) to some /home directory, scrap your /, reinstall and fetch the old configurations from the save directory on /home. 5. How I did it on my machine There was this old ISA bus 386/40 sitting on my shelf that I abandoned two years ago because it no longer cut it. I was planning to turn it into a small X-less server for my household LAN. Here is how I did it: I took that 386 and put 16 MB RAM into it. Added a cheap EIDE disk, the smallest I could get (800 MB) and an ethernet card. Added an old Hercules because I still had a monitor for it. Installed Linux on it and there I have my local NFS, SMB, HTTP, LPD/LPR and NNTP server as well as my mail router and POP3 server. With an additional ISDN card the machine became my TCP/IP router and firewall, too. Most of the disk space on this machine went into the /var directories, /var/spool/mail, /var/spool/news and /var/httpd/html. I put /var on a separate partition and made this one large. There will be almost no users on this machine, so I created no home partition and mounted /home from some other workstation via NFS. Linux without X plus several locally installed utilities will be fine with a 250 MB partition as /. The machine has 16 MB of RAM, but it will be running many servers. 16 MB swap should be in order, 32 MB should be plenty. We are not short on disk space, so the machine will get 32 MB. Out of sentimentality a MS-DOS partition of some 20 MB is kept on it. I decided to import /home from another machine, so the remaining 500+ MB will end up as /var. This is more than sufficient for a household USENET news feed. We get ______________________________________________________________________ Device Mounted on Size /dev/hda1 /dos_c 25 MB /dev/hda2 - (Swapspace) 32 MB /dev/hda3 / 250 MB /dev/hda4 - (Extended Container) 500 MB /dev/hda5 /var 500 MB homeserver:/home /home 1.6 GB ______________________________________________________________________ I am backing up this machine via the network using the tape in homeserver. Since everything on this machine has been installed from CD-ROM all I have to save are some configuration files from /etc, my customized locally installed *.tgz files from /root/Source/Installed and /var/spool/mail as well as /var/httpd/html. I copy these files into a dedicated directory /home/backmeup on homeserver every night, where the regular homeserver backup picks them up.