Re: virtio-blk performance regression and qemu-kvm

[Rusty Russell <rusty@rustcorp.com.au>]

On Tue, 21 Feb 2012 17:45:08 +0100, Dongsu Park <dongsu.park@profitbricks.com> wrote:
> Hi Rusty,
> 
> On 13.02.2012 10:25, Rusty Russell wrote:
> > On Fri, 10 Feb 2012 15:36:39 +0100, Dongsu Park <dongsu.park@profitbricks.com> wrote:
> > > Hi,
> > > 
> > > Recently I observed performance regression regarding virtio-blk,
> > > especially different IO bandwidths between qemu-kvm 0.14.1 and 1.0.
> > > So I want to share the benchmark results, and ask you what the reason
> > > would be.
> > 
> > Interesting.  There are two obvious possibilities here.  One is that
> > qemu has regressed, the other is that virtio_blk has regressed; the new
> > qemu may negotiate new features.  Please do the following in the guest
> > with old and new qemus:
> > 
> > cat /sys/class/block/vdb/device/features
> > 
> > (eg, here that gives: 0010101101100000000000000000100e0).
> 
> I did that on guest VM, using both qemu-kvm 0.14.1 and 1.0.
> (cat /sys/class/block/vdb/device/features)
> 
> using qemu-kvm 0.14.1:
> 
> 0010101101100000000000000000100000000000000000000000000000000000
> 
> using qemu-kvm 1.0:
> 
> 0010101101100000000000000000110000000000000000000000000000000000
> 
> >From my understanding, both of them have the same virtio features.
> Please correct me if I'm wrong.

Well, 1.0 supports event index (feature 29), but that's the only
difference.

This seems very much like a qemu regression.

Thanks,
Rusty.