Re: [Qemu-block] [Qemu-devel] [PATCH 1/2] Add save-snapshot, load-snapshot and delete-snapshot to QAPI

[Dr. David Alan Gilbert]

* Daniel P. Berrange (address@hidden) wrote:
> On Thu, Jan 11, 2018 at 01:46:38PM +0100, Max Reitz wrote:
> > On 2018-01-08 14:52, Eric Blake wrote:
> > > On 01/07/2018 06:23 AM, Richard Palethorpe wrote:
> > >> Add QAPI wrapper functions for the existing snapshot functionality. These
> > >> functions behave the same way as the HMP savevm, loadvm and delvm
> > >> commands. This will allow applications, such as OpenQA, to 
> > >> programmatically
> > >> revert the VM to a previous state with no dependence on HMP or qemu-img.
> > > 
> > > That's already possible; libvirt uses QMP's human-monitor-command to
> > > access these HMP commands programmatically.
> > > 
> > > We've had discussions in the past about what it would take to have
> > > specific QMP commands for these operations; the biggest problem is that
> > > these commands promote the use of internal snapshots, and there are
> > > enough performance and other issues with internal snapshots that we are
> > > not yet ready to commit to a long-term interface for making their use
> > > easier.  At this point, our recommendation is to prefer external 
> > > snapshots.
> > 
> > We already have QMP commands for internal snapshots, though.  Isn't the
> > biggest issue that savevm takes too much time to be a synchronous QMP
> > command?
> 
> Ultimately savevm/loadvm are using much of the migration code internally,
> but are not exposed as URI schemes. Could we perhaps take advantage of
> the internal common layer and define a migration URI scheme
> 
>    snapshot:<name>
> 
> where '<name>' is the name of the internal snapshot in the qcow2 file.

I had wondered about that; I'd just thought of doing the migration
saving to a block device rather than the rest of the snapshot activity around 
it; 
but I guess that's possible.

> Then you could just use the regular migrate QMP commands for loading
> and saving snapshots.  Might need a little extra work on the incoming
> side, since we need to be able to load snapshots, despite QEMU not
> being started with '-incoming defer', but might still be doable ?
> This would theoretically give us progress monitoring, cancellation,
> etc for free.

What actually stops this working other than the sanity check in
migrate_incoming ?

Dave

> 
> 
> Regards,
> Daniel
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--
Dr. David Alan Gilbert / address@hidden / Manchester, UK