Am 13.02.2018 um 12:51 hat Daniel P. Berrangé geschrieben: > On Tue, Feb 13, 2018 at 11:43:55AM +0000, Dr. David Alan Gilbert wrote: > > * Kevin Wolf ([email protected]) 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
Yes, they are non-live, so RAM simply doesn't change and we that's the reason why we currently don't get duplicate pages in the snapshot. But as I understand it, the whole point of migrate snapshot:... would be to make it live, so the potential duplication is something that we need to consider for it. Kevin
