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

[Kevin Wolf]

Am 13.02.2018 um 16:30 hat Daniel P. Berrangé geschrieben:
> On Tue, Feb 13, 2018 at 04:23:21PM +0100, Kevin Wolf wrote:
> > 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().
> 
> The would allow QEMU memory usage to balloon up to x2  RAM, if there was
> slow storage and very fast dirtying rate. I don't think that's viable
> unless there was a cap on how much bounce buffering we would allow before
> just blocking the page faults

Yes, you'd probably want to do this. But anyway, unless the guest is
under really heavy load, allowing just a few MB to be dirtied without
waiting for I/O will make the guest a lot more responsive immediately
after taking a snapshot.

Kevin