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

[Anthony Liguori]

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.

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

Regards,

Anthony Liguori

> Regards,
> Daniel
>