PC World Komputer 1998 July & August/Pcwk78a98.iso/Wtestowe/Clico/UNIX/SAMBA/SMB_SOL2.Z/SMB_SOL2/docs/Speed.txt

Text File | 1997-08-27 | 11.7 KB | 319 lines

Contributor:    Andrew Tridgell
Date:        January 1995
Status:        Current

Subject:    Samba performance issues
============================================================================

This file tries to outline the ways to improve the speed of a Samba server.

COMPARISONS
-----------

The Samba server uses TCP to talk to the client. Thus if you are
trying to see if it performs well you should really compare it to
programs that use the same protocol. The most readily available
programs for file transfer that use TCP are ftp or another TCP based
SMB server.

If you want to test against something like a NT or WfWg server then
you will have to disable all but TCP on either the client or
server. Otherwise you may well be using a totally different protocol
(such as Netbeui) and comparisons may not be valid.

Generally you should find that Samba performs similarly to ftp at raw
transfer speed. It should perform quite a bit faster than NFS,
although this very much depends on your system.

Several people have done comparisons between Samba and Novell, NFS or
WinNT. In some cases Samba performed the best, in others the worst. I
suspect the biggest factor is not Samba vs some other system but the
hardware and drivers used on the various systems. Given similar
hardware Samba should certainly be competitive in speed with other
systems.


OPLOCKS
-------

Oplocks are the way that SMB clients get permission from a server to
locally cache file operations. If a server grants an oplock
(opportunistic lock) then the client is free to assume that it is the
only one accessing the file and it will agressively cache file
data. With some oplock types the client may even cache file open/close
operations. This can give enormous performance benefits.

Samba does not support opportunistic locks because they are very
difficult to do under Unix. Samba can fake them, however, by granting
a oplock whenever a client asks for one. This is controlled using the
smb.conf option "fake oplocks". If you set "fake oplocks = yes" then
you are telling the client that it may agressively cache the file
data.

By enabling this option on all read-only shares or shares that you know
will only be accessed from one client at a time you will see a big
performance improvement on many operations. If you enable this option
on shares where multiple clients may be accessing the files read-write
at the same time you can get data corruption. Use this option
carefully!

This option is disabled by default.

SOCKET OPTIONS
--------------

There are a number of socket options that can greatly affect the
performance of a TCP based server like Samba.

The socket options that Samba uses are settable both on the command
line with the -O option, or in the smb.conf file.

The "socket options" section of the smb.conf manual page describes how
to set these and gives recommendations.

Getting the socket options right can make a big difference to your
performance, but getting them wrong can degrade it by just as
much. The correct settings are very dependent on your local network.

The socket option TCP_NODELAY is the one that seems to make the
biggest single difference for most networks. Many people report that
adding "socket options = TCP_NODELAY" doubles the read performance of
a Samba drive. The best explanation I have seen for this is that the
Microsoft TCP/IP stack is slow in sending tcp ACKs.


READ SIZE
---------

The option "read size" affects the overlap of disk reads/writes with
network reads/writes. If the amount of data being transferred in
several of the SMB commands (currently SMBwrite, SMBwriteX and
SMBreadbraw) is larger than this value then the server begins writing
the data before it has received the whole packet from the network, or
in the case of SMBreadbraw, it begins writing to the network before
all the data has been read from disk.

This overlapping works best when the speeds of disk and network access
are similar, having very little effect when the speed of one is much
greater than the other.

The default value is 16384, but very little experimentation has been
done yet to determine the optimal value, and it is likely that the best
value will vary greatly between systems anyway. A value over 65536 is
pointless and will cause you to allocate memory unnecessarily.


MAX XMIT
--------

At startup the client and server negotiate a "maximum transmit" size,
which limits the size of nearly all SMB commands. You can set the
maximum size that Samba will negotiate using the "max xmit = " option
in smb.conf. Note that this is the maximum size of SMB request that
Samba will accept, but not the maximum size that the *client* will accept.
The client maximum receive size is sent to Samba by the client and Samba
honours this limit.

It defaults to 65536 bytes (the maximum), but it is possible that some
clients may perform better with a smaller transmit unit. Trying values
of less than 2048 is likely to cause severe problems.

In most cases the default is the best option.


LOCKING
-------

By default Samba does not implement strict locking on each read/write
call (although it did in previous versions). If you enable strict
locking (using "strict locking = yes") then you may find that you
suffer a severe performance hit on some systems.

The performance hit will probably be greater on NFS mounted
filesystems, but could be quite high even on local disks.


SHARE MODES
-----------

Some people find that opening files is very slow. This is often
because of the "share modes" code needed to fully implement the dos
share modes stuff. You can disable this code using "share modes =
no". This will gain you a lot in opening and closing files but will
mean that (in some cases) the system won't force a second user of a
file to open the file read-only if the first has it open
read-write. For many applications that do their own locking this
doesn't matter, but for some it may. Most Windows applications
depend heavily on "share modes" working correctly and it is
recommended that the Samba share mode support be left at the
default of "on".

The share mode code in Samba has been re-written in the 1.9.17
release following tests with the Ziff-Davis NetBench PC Benchmarking
tool. It is now believed that Samba 1.9.17 implements share modes
similarly to Windows NT.

NOTE: In the most recent versions of Samba there is an option to use
shared memory via mmap() to implement the share modes. This makes
things much faster. See the Makefile for how to enable this.


LOG LEVEL
---------

If you set the log level (also known as "debug level") higher than 2
then you may suffer a large drop in performance. This is because the
server flushes the log file after each operation, which can be very
expensive.


WIDE LINKS
----------

The "wide links" option is now enabled by default, but if you disable
it (for better security) then you may suffer a performance hit in
resolving filenames. The performance loss is lessened if you have
"getwd cache = yes", which is now the default.


READ RAW
--------

The "read raw" operation is designed to be an optimised, low-latency
file read operation. A server may choose to not support it,
however. and Samba makes support for "read raw" optional, with it
being enabled by default.

In some cases clients don't handle "read raw" very well and actually
get lower performance using it than they get using the conventional
read operations.

So you might like to try "read raw = no" and see what happens on your
network. It might lower, raise or not affect your performance. Only
testing can really tell.


WRITE RAW
---------

The "write raw" operation is designed to be an optimised, low-latency
file write operation. A server may choose to not support it,
however. and Samba makes support for "write raw" optional, with it
being enabled by default.

Some machines may find "write raw" slower than normal write, in which
case you may wish to change this option.

READ PREDICTION
---------------

Samba can do read prediction on some of the SMB commands. Read
prediction means that Samba reads some extra data on the last file it
read while waiting for the next SMB command to arrive. It can then
respond more quickly when the next read request arrives.

This is disabled by default. You can enable it by using "read
prediction = yes".

Note that read prediction is only used on files that were opened read
only.

Read prediction should particularly help for those silly clients (such
as "Write" under NT) which do lots of very small reads on a file.

Samba will not read ahead more data than the amount specified in the
"read size" option. It always reads ahead on 1k block boundaries.


MEMORY MAPPING
--------------

Samba supports reading files via memory mapping them. One some
machines this can give a large boost to performance, on others it
makes not difference at all, and on some it may reduce performance.

To enable you you have to recompile Samba with the -DUSE_MMAP option
on the FLAGS line of the Makefile.

Note that memory mapping is only used on files opened read only, and
is not used by the "read raw" operation. Thus you may find memory
mapping is more effective if you disable "read raw" using "read raw =
no".


SLOW CLIENTS
------------

One person has reported that setting the protocol to COREPLUS rather
than LANMAN2 gave a dramatic speed improvement (from 10k/s to 150k/s).

I suspect that his PC's (386sx16 based) were asking for more data than
they could chew. I suspect a similar speed could be had by setting
"read raw = no" and "max xmit = 2048", instead of changing the
protocol. Lowering the "read size" might also help.


SLOW LOGINS
-----------

Slow logins are almost always due to the password checking time. Using
the lowest practical "password level" will improve things a lot. You
could also enable the "UFC crypt" option in the Makefile.

CLIENT TUNING
-------------

Often a speed problem can be traced to the client. The client (for
example Windows for Workgroups) can often be tuned for better TCP
performance.

See your client docs for details. In particular, I have heard rumours
that the WfWg options TCPWINDOWSIZE and TCPSEGMENTSIZE can have a
large impact on performance.

Also note that some people have found that setting DefaultRcvWindow in
the [MSTCP] section of the SYSTEM.INI file under WfWg to 3072 gives a
big improvement. I don't know why.

My own experience wth DefaultRcvWindow is that I get much better
performance with a large value (16384 or larger). Other people have
reported that anything over 3072 slows things down enourmously. One
person even reported a speed drop of a factor of 30 when he went from
3072 to 8192. I don't know why.

It probably depends a lot on your hardware, and the type of unix box
you have at the other end of the link.


MY RESULTS
----------

Some people want to see real numbers in a document like this, so here
they are. I have a 486sx33 client running WfWg 3.11 with the 3.11b
tcp/ip stack. It has a slow IDE drive and 20Mb of ram. It has a SMC
Elite-16 ISA bus ethernet card. The only WfWg tuning I've done is to
set DefaultRcvWindow in the [MSTCP] section of system.ini to 16384. My
server is a 486dx3-66 running Linux. It also has 20Mb of ram and a SMC
Elite-16 card. You can see my server config in the examples/tridge/
subdirectory of the distribution.

I get 490k/s on reading a 8Mb file with copy.
I get 441k/s writing the same file to the samba server.

Of course, there's a lot more to benchmarks than 2 raw throughput
figures, but it gives you a ballpark figure.

I've also tested Win95 and WinNT, and found WinNT gave me the best
speed as a samba client. The fastest client of all (for me) is
smbclient running on another linux box. Maybe I'll add those results
here someday ...


COMMENTS
--------

If you've read this far then please give me some feedback! Which of
the above suggestions worked for you?

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