On Tue, Feb 13, 2018 at 03:43:10PM +0100, Kevin Wolf wrote: > Am 13.02.2018 um 15:30 hat Roman Kagan geschrieben: > > On Tue, Feb 13, 2018 at 11:50:24AM +0100, Kevin Wolf wrote: > > > Am 11.01.2018 um 14:04 hat Daniel P. Berrange geschrieben: > > > > 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 [...] > > > > This means the point in time where the guest state is snapshotted is not > > when the command is issued, but any unpredictable amount of time later. > > > > I'm not sure this is what a user expects. > > I don't think it's necessarily a big problem as long as you set the > expectations right, but good point anyway. > > > A better approach for the save part appears to be to stop the vcpus, > > dump the device state, resume the vcpus, and save the memory contents > > in the background, prioritizing the old copies of the pages that > > change. > > So basically you would let the guest fault whenever it writes to a page > that is not saved yet, and then save it first before you make the page > writable again? Essentially blockdev-backup, except for RAM.
The page fault servicing will be delayed by however long it takes to write the page to underling storage, which could be considerable with non-SSD. So guest performance could be significantly impacted on slow storage with high dirtying rate. On the flip side it gurantees a live snapshot would complete in finite time which is good. 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 :|
