Re: [Qemu-block] [PATCH v2 06/22] block: Add "supports_stats" field to BlockStats

[Stefan Hajnoczi]

On Thu, Oct 15, 2015 at 03:06:19PM +0200, Alberto Garcia wrote:
> On Tue 13 Oct 2015 05:38:30 PM CEST, Stefan Hajnoczi wrote:
> 
> >> query-blockstats always returns a BlockDeviceStats structure for each
> >> BDS, regardless of whether it implements accounting or not. Since the
> >> field is mandatory there's no way to tell that from the values alone.
> >> 
> >> This field solves that problem by indicating which BDSs support I/O
> >> accounting.
> >
> > I'm not sure I understand the problem.
> >
> > If I/O accounting isn't being used then all fields will be 0?
> 
> Yes, but there's no way to tell if that happens because I/O accounting
> is not supported or because there hasn't been any I/O yet.
> 
> There's one additional problem: this patch assumes that accounting is
> supported if this BDS is attached to a BlockBackend. But we don't know
> if the device model supports accounting or not, I still need to figure
> out what's the best way to do it.

Is there a corresponding libvirt patch or why does it matter whether the
QMP client can detect whether blockstats are available?

Stefan