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

[Stefan de Konink <stefan@xxxxxxxxx>]

tap or loop?

On Tue, 14 Oct 2008, Frederik Himpe wrote:

> 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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> http://lists.xensource.com/xen-users
>
>


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