[Qemu-devel] Re: [PATCH 1/2] block: Add wr_highest_sector blockstat

[Luiz Capitulino <lcapitulino@redhat.com>]

On Wed, 28 Apr 2010 11:01:12 -0500
Anthony Liguori <anthony@codemonkey.ws> wrote:

> On 04/28/2010 10:56 AM, Kevin Wolf wrote:
> > This adds the wr_highest_sector blockstat which implements what is generally
> > known as the high watermark. It is the highest offset of a sector written to
> > the respective BlockDriverState since it has been opened.
> >
> > The query-blockstat QMP command is extended to add this value to the result,
> > and also to add the statistics of the underlying protocol in a new "parent"
> > field. Note that to get the "high watermark" of a qcow2 image, you need to look
> > into the wr_highest_sector field of the parent (which can be a file, a
> > host_device, ...). The wr_highest_sector of the qcow2 BlockDriverState itself
> > is the highest offset on the _virtual_ disk that the guest has written to.
> >
> > Signed-off-by: Kevin Wolf<kwolf@redhat.com>
> >    
> 
> I see, you did print out stats for each layer.
> 
> I don't think we should take 2/2.  I don't mind QMP having more features 
> than the user monitor.

 I don't either, but Kevin has said to me that this information is also good
for the user Monitor.

 The real question here is whether or not we're going to stop supporting
stability for the user Monitor and if so, when we'll break it.

 An arguable reasonable policy would be to try to maintain stability for
existing commands. In this specific case, 'info blockstats' is used by
libvirt afaik. So breaking it would mean that older libvirt versions won't
be able to talk to newer qemu (taking libvirt just as real known example).