Re: [Qemu-block] [Qemu-devel] [PATCH 1/2] Add save-snapshot, load-snapshot and delete-snapshot to QAPI

[Daniel P . Berrangé]

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
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