On Thu, Jan 11, 2018 at 01:23:05PM +0000, Dr. David Alan Gilbert wrote: > * Daniel P. Berrange (berra...@redhat.com) 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 -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
