* Kevin Wolf (kw...@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 / dgilb...@redhat.com / Manchester, UK
