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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* 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.

Dave

> Kevin
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK