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

[Dr. David Alan Gilbert]

* Daniel P. Berrangé (address@hidden) wrote:
> On Tue, Feb 13, 2018 at 03:45:21PM +0100, Kevin Wolf wrote:
> > Am 13.02.2018 um 15:36 hat Daniel P. Berrangé geschrieben:
> > > On Tue, Feb 13, 2018 at 05:30:02PM +0300, Roman Kagan wrote:
> > > > 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.
> > > >
> > > > 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.
> > > > No multiple copies of the same page would have to be saved so the stream
> > > > format would be fine.  For the load part the usual inmigrate should
> > > > work.
> > > 
> > > No, that's policy decision that doesn't matter from QMP pov. If the mgmt
> > > app wants the snapshot to be wrt to the initial time, it can simply
> > > invoke the "stop" QMP command before doing the live migration and
> > > "cont" afterwards.
> > 
> > That would be non-live. I think Roman means a live snapshot that saves
> > the state at the beginning of the operation. Basically the difference
> > between blockdev-backup (state at the beginning) and blockdev-mirror
> > (state at the end), except for a whole VM.
> 
> That doesn't seem practical unless you can instantaneously write out
> the entire guest RAM to disk without blocking, or can somehow snapshot
> the RAM so you can write out a consistent view of the original RAM,
> while the guest continues to dirty RAM pages.

People have suggested doing something like that with userfault write
mode; but the same would also be doable just by write protecting the
whole of RAM and then following the faults.

Dave

> Regards,
> Daniel
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--
Dr. David Alan Gilbert / address@hidden / Manchester, UK