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From: Markus Armbruster <armbru@redhat.com>
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Date: Thu, 28 Aug 2014 19:14:07 +0200
In-Reply-To: <20140828150044.GE28789@irqsave.net> (=?utf-8?Q?=22Beno=C3=AE?=
	=?utf-8?Q?t?= Canet"'s message of
	"Thu, 28 Aug 2014 17:00:44 +0200")
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Subject: Re: [Qemu-devel] block IO latency tracker without using QMP socket.
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To: =?utf-8?Q?Beno=C3=AEt?= Canet <benoit.canet@irqsave.net>
Cc: Stefan Hajnoczi <stefanha@gmail.com>, qemu-devel <qemu-devel@nongnu.org>, Anshul Makkar <anshul.makkar@profitbricks.com>

Beno=C3=AEt Canet <benoit.canet@irqsave.net> writes:

> The Thursday 28 Aug 2014 =C3=A0 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 bet=
ween
>> > start of IO and end of IO.
>> > (firing my latency tracker from function BlockDriverAIOCB *raw_aio_sub=
mit()
>> > raw-posix.c when job is submitted).
>> >=20
>> > The latency data per QEMU process will be written to shared memory and=
 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 bloc=
ked
>> > by some other process in our subsystem.
>> >=20
>> > Just want a suggestion whether my approach is correct given the constr=
aint
>> > 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
> very useful.

For what it's worth, query-blockstats already reports the sum of certain
latencies.