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

[Alberto Garcia]

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.

Berto