Re: [Qemu-devel] block IO latency tracker without using QMP socket.

[Markus Armbruster <armbru@redhat.com>]

Benoît Canet <benoit.canet@irqsave.net> writes:

> The Thursday 28 Aug 2014 à 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.
>> > 
>> > As obvious,  I am calculating the latency by tracking the interval between
>> > start of IO and end of IO.
>> > (firing my latency tracker from function BlockDriverAIOCB *raw_aio_submit()
>> > raw-posix.c when job is submitted).
>> > 
>> > 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.
>> > 
>> > Can't use "info blockstats" QMP command as qmp socket is used and blocked
>> > by some other process in our subsystem.
>> > 
>> > Just want a suggestion whether my approach is correct given the constraint
>> > that I can't use qmp socket or if any alternative is possible.
>> 
>> Simplest option: use iostat(1) inside the guest.
>> 
>> Or you can use tracing tools to measure block I/O request latency.
>> 
>> An example from a few years ago:
>> http://www.linux-kvm.org/page/Virtio/Block/Latency
>> 
>> 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.