[Xen-users] Xen, NFS performance, rsize, wsize and MTU

[Richard Jones <rich@xxxxxxxxxxx>]

We have a Xen 3.0 / Linux kernel 2.6.15 machine with the domU's
configured for shared /home directories.  One of the domU's is an NFS
server, exporting /home, and the other domU's all mount this.

It all functions fine, but it's pretty slow.

I tried untarring a recent Linux kernel tarball.  On the NFS server
domU this takes about 25 seconds.  I didn't wait long enough to find
out how long it was going to take on the NFS client domU's - I killed
it after 15 minutes.

I was going to try compiling Linux kernels on the various domU's to
test for stability, but that's excruciatingly slow too.

Following the instructions here:

http://nfs.sourceforge.net/nfs-howto/performance.html

I've been playing with rsize, wsize and sync mount options.  As far as
I can see, any rsize/wsize other than the default is slower than the
default choice (I couldn't find out what it is, but it _seems_ to be
1K/1K).  The 'sync' mount option has further catastrophic effects on
write performance.

I also notice that the MTU chosen on the eth0 virtual interfaces is
1500 bytes.  This seems to make no sense, because the purpose behind
the MTU is to do with the physical characteristics of ethernet itself,
but here we've got an entirely virtual bridge setup.  Surely I should
choose an MTU as large as possible (ie. 64K)?

Before I start to look into changing MTU and further modifying
rsize/wsize, has anyone got any quick tips on how to make my NFS
config faster?

Rich.

-- 
Richard Jones, CTO Merjis Ltd.
Merjis - web marketing and technology - http://merjis.com
Team Notepad - intranets and extranets for business - http://team-notepad.com

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