Am 13.02.2018 um 15:58 hat Daniel P. Berrangé geschrieben: > 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.
You can just use a bounce buffer for writing out the old page. Then the VM is only stopped for the duration of a malloc() + memcpy(). Kevin
