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

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Message-ID: <4E519546.1050406@codemonkey.ws>
Date:	Sun, 21 Aug 2011 18:31:18 -0500
From:	Anthony Liguori <anthony@...emonkey.ws>
To:	Sasha Levin <levinsasha928@...il.com>
CC:	Avi Kivity <avi@...hat.com>, 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,
	Pekka Enberg <penberg@...nel.org>, Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] virtio-blk: Add stats VQ to collect information about
 devices

On 08/18/2011 11:29 AM, Sasha Levin wrote:
> On Thu, 2011-08-18 at 08:10 -0700, Avi Kivity wrote:
>> 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?
>>
>
> I assumed that the time the request spends in the virtio layer is
> (somewhat) significant, specially since that this is something that adds
> up over time.
>
> Merge count can be useful for several testing scenarios (I'll describe
> the reasoning behind this patch below).
>
>>>
>>> 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 was actually planning on submitting another patch that would add
> something similar into virtio-net. My plan was to enable collecting
> statistics regarding memory, network and disk usage in a simple manner
> without accessing guests.

Why not just add an interface that lets you read files from a guest 
either via a guest agent (like qemu-ga) or a purpose built PV device?

That would let you access the guest's full sysfs which seems to be quite 
a lot more useful long term than adding a bunch of specific interfaces.

Regards,

Anthony Liguori
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