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

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

On Tue, Feb 13, 2018 at 11:43:55AM +0000, Dr. David Alan Gilbert wrote:
> * Kevin Wolf (kwolf@redhat.com) wrote:
> > Am 11.01.2018 um 14:04 hat Daniel P. Berrange geschrieben:
> > > 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.
> > 
> > Let's include a node-name there, please. QEMU automagically deciding
> > where to store the VM state is one of the major problems of the HMP
> > interface.
> > 
> > And while we're at it, we can make it more future-proof by allowing to
> > specify arbitrary options:
> > 
> >     snapshot:node=<node-name>,name=<snapshot-name>
> > 
> > That would allow us to add something like compressed=on|off later.
> > Actually, compressed VM state sounds pretty nice. Why don't we have this
> > yet? The qcow2 format already provides everything you need for it.
> > 
> > > Then you could just use the regular migrate QMP commands for loading
> > > and saving snapshots.
> > 
> > Yes, you could. I think for a proper implementation you would want to do
> > better, though. Live migration provides just a stream, but that's not
> > really well suited for snapshots. When a RAM page is dirtied, you just
> > want to overwrite the old version of it in a snapshot, you don't want to
> > waste space by keeping both the old and the current version of the page
> > content in the file.
> 
> The current snapshots are run with the CPU paused aren't they?  They
> share exactly the same RAM saving code as migration.

The commands block the monitor too, so I always assumed they were non-live

Regards,
Daniel
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