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

[Luiz Capitulino <lcapitulino@redhat.com>]

On Wed, 28 Apr 2010 12:47:49 -0500
Anthony Liguori <anthony@codemonkey.ws> wrote:

> On 04/28/2010 12:04 PM, Luiz Capitulino wrote:
> > 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).
> >    
> 
> I think we should try our best to maintain compatibility.  In this case, 
> this change would break any non-QMP version of libvirt so it would be 
> pretty painful for users.  That's why I'm inclined to not take.
> 
> It would be reasonable to add a new info command for the user monitor if 
> the functionality is desirable.

 Seems a good solution to me too.