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

[Daniel P. Berrange]

On Thu, Jan 11, 2018 at 01:23:05PM +0000, Dr. David Alan Gilbert wrote:
> * 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.

One possible gotcha is whether the current savevm/loadvm QEMUFile impl
actually does non-blocking I/O properly. eg same reason why we don't
support a plain  file:<path> protocol - POSIX I/O on plain files doesn't
honour O_NONBLOCK.  The block layer does AIO though, so we might be OK,
depending on which block layer APIs the QEMUFile impl uses. I've not
looked at the code recently though.

> 
> > 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 ?

No idea really - not looked closely at the code implications.

Regards,
Daniel
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