From mboxrd@z Thu Jan  1 00:00:00 1970
From: Rusty Russell <rusty@rustcorp.com.au>
Subject: Re: virtio-blk performance regression and qemu-kvm
Date: Wed, 22 Feb 2012 08:46:41 +1030
Message-ID: <87obsr6ddi.fsf@rustcorp.com.au>
References: <20120210143639.GA17883@gmail.com> <87sjifd2sw.fsf@rustcorp.com.au> <20120221164508.GB950@gmail.com>
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To: Dongsu Park <dongsu.park@profitbricks.com>
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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.