[Xen-users] very slow I/O performance in domU

[Frederik Himpe <fhimpe@xxxxxxxxxx>]

I've got a Debian Lenny Dom0 with Debian's 2.6.26 Xen paravirt_ops kernel 
and a Debian Lenny DomU. xen-hypervisor is Debian's 3.2.1 package. The 
dom0 has two dual core Opteron CPUs without hardware virtualisation 
support.

In the domU I/O performance is very bad. In the dom0, I get about 46 MB/s:

# dd if=/dev/zero of=/root/zeroes bs=20M count=20
20+0 records in
20+0 records out
419430400 bytes (419 MB) copied, 9.12234 s, 46.0 MB/s

While the domU only gets 6:

Last login: Tue Oct 14 07:46:10 2008 from 192.168.0.129

# dd if=/dev/zero of=/root/zeroes bs=20M count=20
20+0 records in
20+0 records out
419430400 bytes (419 MB) copied, 70.1395 s, 6.0 MB/s

The domU only has 384 MB of RAM, while the dom0 has 4 GB, but as almost 
nothing is running in the domU and its cache is pretty big, I doubt this 
can explain this enormous difference in I/O performance:

# free
             total       used       free     shared    buffers     cached
Mem:        393432     366800      26632          0       2420     328444
-/+ buffers/cache:      35936     357496
Swap:       131064         20     131044

Both the dom0 and domU are using the ext3 file system on LVM logical 
volumes on a 3Ware hardware RAID 5.

Any idea what I could try to improve the performance?

-- 
Frederik Himpe


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