Re: bad virtio disk performance

[john cooper <john.cooper@third-harmonic.com>]

Lucas Nussbaum wrote:
> Hi,
> 
> I'm experiencing bad disk I/O performance using virtio disks.
> 
> I'm using Linux 2.6.29 (host & guest), kvm 84 userspace.
> On the host, and in a non-virtio guest, I get ~120 MB/s when writing
> with dd (the disks are fast RAID0 SAS disks).

Could you provide detail of the exact type and size
of i/o load you were creating with dd?

Also the full qemu cmd line invocation in both
cases would be useful.

> In a guest with a virtio disk, I get at most ~32 MB/s.

Which non-virtio interface was used for the
comparison?

> The rest of the setup is the same. For reference, I'm running kvm -drive
> file=/tmp/debian-amd64.img,if=virtio.
> 
> Is such performance expected? What should I check?

Not expected, something is awry.

blktrace(8) run on the host will shed some light
on the type of i/o requests issued by qemu in both
cases.

-john


-- 
john.cooper@third-harmonic.com