linux-kernel - Re: [PATCH] virtio-blk: Add stats VQ to collect information about devices

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Message-ID: <4E4D2B6D.1040908@redhat.com>
Date:	Thu, 18 Aug 2011 08:10:37 -0700
From:	Avi Kivity <avi@...hat.com>
To:	Sasha Levin <levinsasha928@...il.com>
CC:	linux-kernel@...r.kernel.org,
	Rusty Russell <rusty@...tcorp.com.au>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org
Subject: Re: [PATCH] virtio-blk: Add stats VQ to collect information about
 devices

On 08/17/2011 09:38 PM, Sasha Levin wrote:
> On Wed, 2011-08-17 at 16:00 -0700, Avi Kivity wrote:
> >  On 08/16/2011 12:47 PM, Sasha Levin wrote:
> >  >  This patch adds support for an optional stats vq that works similary to the
> >  >  stats vq provided by virtio-balloon.
> >  >
> >  >  The purpose of this change is to allow collection of statistics about working
> >  >  virtio-blk devices to easily analyze performance without having to tap into
> >  >  the guest.
> >  >
> >  >
> >
> >  Why can't you get the same info from the host?  i.e. read sectors?
>
> Some of the stats you can collect from the host, but some you can't.
>
> The ones you can't include all the timing statistics and the internal
> queue statistics (read/write merges).

Surely you can time the actual amount of time the I/O takes?  It doesn't 
account for the virtio round-trip, but does it matter?

Why is the merge count important for the host?

>
> The idea behind providing all of the stats on the stats vq (which is
> basically what you see in '/dev/block/[device]/stats') is to give a
> consistent snapshot of the state of the device.
>
>

What can you do with it?

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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