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Date: Wed, 14 Sep 2011 15:17:17 +0100
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From: Stefan Hajnoczi <stefanha@gmail.com>
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Subject: Re: [Qemu-devel] Why qemu write/rw speed is so low?
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To: Zhi Yong Wu <zwu.kernel@gmail.com>
Cc: kwolf@redhat.com, aliguro@us.ibm.com, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>, Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>, qemu-devel@nongnu.org, ryanh@us.ibm.com

On Wed, Sep 14, 2011 at 3:42 AM, Zhi Yong Wu <zwu.kernel@gmail.com> wrote:
> Log for bps=3D((10 * 1024 * 1024)).

Okay, I think this data shows that I/O limits is too aggressive.
There seems to be some "overhead" amount so the guest is never able to
reach its bps limit:

> test: (g=3D0): rw=3Dwrite, bs=3D64K-64K/64K-64K, ioengine=3Dlibaio, iodep=
th=3D1
> =A0WRITE: io=3D51,200KB, aggrb=3D7,073KB/s, minb=3D7,243KB/s, maxb=3D7,24=
3KB/s,

> test: (g=3D0): rw=3Dwrite, bs=3D128K-128K/128K-128K, ioengine=3Dlibaio, i=
odepth=3D1
> =A0WRITE: io=3D51,200KB, aggrb=3D10,112KB/s, minb=3D10,355KB/s,

> test: (g=3D0): rw=3Dwrite, bs=3D256K-256K/256K-256K, ioengine=3Dlibaio, i=
odepth=3D1
> =A0WRITE: io=3D51,200KB, aggrb=3D8,882KB/s, minb=3D9,095KB/s, maxb=3D9,09=
5KB/s,

bs=3D128 KB worked nicely.  The 64 KB and 256 KB cases don't look so good.

I worry a little that the benchmark duration is quite short so a
fluctuation would affect the results more than if the benchmark
duration was extended to 30 secs or 1 minute.

Zhi Yong: Do you get similar results each time you run this benchmark
or do they vary by more than +/- 512 KB?  If the results are stable
and the benchmark is able to exceed 10 MB/s when running without I/O
throttling, then it's important to figure out why the guest isn't
achieving 10 MB/s.

Stefan