Re: [PATCH] virtio-blk: Add stats VQ to collect information aboutdevices

[Anthony Liguori]

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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/