Chapter 39. Samba Performance Tuning

Chapter 39. Samba Performance Tuning
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Paul Cochrane

Dundee Limb Fitting Centre

<paulc@dth.scot.nhs.uk>

Jelmer R. Vernooij

The Samba Team

<jelmer@samba.org>

John H. Terpstra

Samba Team

<jht@samba.org>

Table of Contents

Comparisons
Socket Options
Read Size
Max Xmit
Log Level
Read Raw
Write Raw
Slow Logins
Client Tuning
Samba Performance Problem Due to Changing Linux Kernel
Corrupt tdb Files

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 an NT or Windows for Workgroups 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 depends on your system. -

-Several people have done comparisons between Samba and Novell, NFS or -Windows NT. In some cases Samba performed the best, in others the worst. I -suspect the biggest factor is not Samba versus 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. -

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 correct 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 little effect when the speed of one is much -greater than the other. -

-The default value is 16384, but little experimentation has been -done as 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 requests 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 -honors 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. -

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 quite -expensive. -

Read Raw

-The read raw operation is designed to be an optimized, 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 do not 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 effect your performance. Only -testing can really tell. -

Write Raw

-The write raw operation is designed to be an optimized, 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. -

Slow Logins

-Slow logins are almost always due to the password checking time. Using -the lowest practical password level will improve things. -

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. Check the sections on the various clients in -. -

Samba Performance Problem Due to Changing Linux Kernel

-A user wrote the following to the mailing list: -

-I am running Gentoo on my server and Samba 2.2.8a. Recently -I changed kernel version from linux-2.4.19-gentoo-r10 to -linux-2.4.20-wolk4.0s. And now I have a performance issue with Samba. -Many of you will probably say, “Move to vanilla sources!” -Well, I tried that and it didn't work. I have a 100mb LAN and two computers (Linux and -Windows 2000). The Linux server shares directories with DivX files, the client -(Windows 2000) plays them via LAN. Before when I was running the 2.4.19 kernel -everything was fine, but now movies freeze and stop. I tried moving -files between the server and Windows and it is terribly slow. -

-The answer he was given is: -

-Grab the mii-tool and check the duplex settings on the NIC. -My guess is that it is a link layer issue, not an application -layer problem. Also run ifconfig and verify that the framing -error, collisions, and so on, look normal for ethernet. -

Corrupt tdb Files

-Our Samba PDC server has been hosting three TB of data to our 500+ users -[Windows NT/XP] for the last three years using Samba without a problem. -Today all shares went very slow. Also the main smbd kept -spawning new processes so we had 1600+ running smbd's (normally we avg. 250). -It crashed the SUN E3500 cluster twice. After a lot of searching, I -decided to rm /var/locks/*.tdb. Happy again. -

-Question: Is there any method of keeping the *.tdb files in top condition or -how can I detect early corruption? -

-Answer: Yes, run tdbbackup each time after stopping nmbd and before starting nmbd. -

-Question: What I also would like to mention is that the service latency seems -a lot lower than before the locks cleanup. Any ideas on keeping it top notch? -

-Answer: Yes. Same answer as for previous question! -

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