Re: [Qemu-devel] Re: KVM call minutes for June 8

["Daniel P. Berrange" <berrange@redhat.com>]

On Wed, Jun 09, 2010 at 11:22:55AM -0500, Anthony Liguori wrote:
> On 06/09/2010 10:31 AM, Daniel P. Berrange wrote:
> >>  However, libvirt was counting on this feature and on the snapshot 
> >>  commands
> >>to switch from the text Monitor. We have two options:
> >>
> >>  1. Ask them to wait one more release (not so good for us)
> >>  2. Try to find a way to have those features in for 0.13
> >>
> >>  Daniel has commented to me that making the snapshot commands synchronous
> >>for 0.13 wouldn't be that bad, what do you think?
> >>     
> >The thought is that changing a command from synchronous to asynchronous is
> >not an ABI incompatible change. An existing app simply won't know to take
> >advantage of the new possibilities that async commands offer.
> >   
> 
> It's not QMP that's the major issue with savevm.  The major issue is 
> actually the way snapshots are saved in qcow2.  You need to know the 
> size of the snapshot prior to creating the snapshot which is not 
> possible if you want to make snapshots live.  In order to support this 
> functionality, we'll probably have to introduce a new snapshot format in 
> qcow2.
> 
> Honestly, I would recommend not supporting snapshots in libvirt until we 
> did the work to make snapshots live.  Snapshots can cause guests to 
> generate errors as it stands today and fixing that will almost certainly 
> result in backwards compatibility considerations.

Libvirt already supports savevm in the text mode monitor, hence the desire
to have it work in QMP too.

> If we did put savevm in QMP today, we would need to provide an interface 
> that let applications detect when we eventually fix this.

The problem you describe sounds like an internal qcow2 implementation
detail. The savevm related commands & their parameters don't have anything
that's inherantly tied to qcow2, so why would it affect QMP if qcow2 format
changed internally ?

Regards,
Daniel
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