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Date: Thu, 28 Aug 2014 17:00:44 +0200
From: =?iso-8859-1?Q?Beno=EEt?= Canet <benoit.canet@irqsave.net>
Message-ID: <20140828150044.GE28789@irqsave.net>
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	<20140828123635.GQ26741@stefanha-thinkpad.redhat.com>
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Subject: Re: [Qemu-devel] block IO latency tracker without using QMP socket.
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To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: Anshul Makkar <anshul.makkar@profitbricks.com>, qemu-devel <qemu-devel@nongnu.org>

The Thursday 28 Aug 2014 =E0 13:36:35 (+0100), Stefan Hajnoczi wrote :
> On Wed, Aug 27, 2014 at 04:58:12PM +0200, Anshul Makkar wrote:
> > I am writing a block IO latency tracker.
> >=20
> > As obvious,  I am calculating the latency by tracking the interval be=
tween
> > start of IO and end of IO.
> > (firing my latency tracker from function BlockDriverAIOCB *raw_aio_su=
bmit()
> > raw-posix.c when job is submitted).
> >=20
> > The latency data per QEMU process will be written to shared memory an=
d then
> > another app uses this shared memory to read the data. That's a simple
> > architecture.
> >=20
> > Can't use "info blockstats" QMP command as qmp socket is used and blo=
cked
> > by some other process in our subsystem.
> >=20
> > Just want a suggestion whether my approach is correct given the const=
raint
> > that I can't use qmp socket or if any alternative is possible.
>=20
> Simplest option: use iostat(1) inside the guest.
>=20
> Or you can use tracing tools to measure block I/O request latency.
>=20
> An example from a few years ago:
> http://www.linux-kvm.org/page/Virtio/Block/Latency
>=20
> It depends what you are trying to achieve.

Stefan: ProfitBrick is a cloud provider.

This means they could prepare AMI (Virtual machine images) with stuff in =
it but
they never would access the shell of a running VM.

What happen in the guest after the AMI instanciation into a running VM is=
 the
end user business and the cloud provider should not violate the end user =
territory.

I also agree that having QEMU collecting latencies statistics would be ve=
ry useful.

Best regards

Beno=EEt

>=20
> Stefan