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Re: [Qemu-block] [Qemu-devel] [PATCH 1/2] Add save-snapshot, load-snapsh

From: Kevin Wolf
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH 1/2] Add save-snapshot, load-snapshot and delete-snapshot to QAPI
Date: Tue, 13 Feb 2018 15:45:21 +0100
User-agent: Mutt/1.9.1 (2017-09-22)

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.


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