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

[Stefan Hajnoczi]

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

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