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

From: "Daniel P. Berrangé" <berrange@redhat.com>
To: Kevin Wolf <kwolf@redhat.com>
Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>,
	Max Reitz <mreitz@redhat.com>,
	Richard Palethorpe <richiejp@f-m.fm>,
	Qemu-block <qemu-block@nongnu.org>,
	quintela@redhat.com, armbru@redhat.com, qemu-devel@nongnu.org,
	rpalethorpe@suse.com
Subject: Re: [Qemu-devel] [Qemu-block]  [PATCH 1/2] Add save-snapshot, load-snapshot and delete-snapshot to QAPI
Date: Tue, 13 Feb 2018 13:25:11 +0000	[thread overview]
Message-ID: <20180213132511.GL573@redhat.com> (raw)
In-Reply-To: <20180213132000.GG5083@localhost.localdomain>

On Tue, Feb 13, 2018 at 02:20:00PM +0100, Kevin Wolf wrote:
> Am 13.02.2018 um 12:51 hat Daniel P. Berrangé geschrieben:
> > On Tue, Feb 13, 2018 at 11:43:55AM +0000, Dr. David Alan Gilbert wrote:
> > > * Kevin Wolf (kwolf@redhat.com) wrote:
> > > > Am 11.01.2018 um 14:04 hat Daniel P. Berrange geschrieben:
> > > > > On Thu, Jan 11, 2018 at 01:46:38PM +0100, Max Reitz wrote:
> > > > > > On 2018-01-08 14:52, Eric Blake wrote:
> > > > > > > On 01/07/2018 06:23 AM, Richard Palethorpe wrote:
> > > > > > >> Add QAPI wrapper functions for the existing snapshot functionality. These
> > > > > > >> functions behave the same way as the HMP savevm, loadvm and delvm
> > > > > > >> commands. This will allow applications, such as OpenQA, to programmatically
> > > > > > >> revert the VM to a previous state with no dependence on HMP or qemu-img.
> > > > > > > 
> > > > > > > That's already possible; libvirt uses QMP's human-monitor-command to
> > > > > > > access these HMP commands programmatically.
> > > > > > > 
> > > > > > > We've had discussions in the past about what it would take to have
> > > > > > > specific QMP commands for these operations; the biggest problem is that
> > > > > > > these commands promote the use of internal snapshots, and there are
> > > > > > > enough performance and other issues with internal snapshots that we are
> > > > > > > not yet ready to commit to a long-term interface for making their use
> > > > > > > easier.  At this point, our recommendation is to prefer external snapshots.
> > > > > > 
> > > > > > We already have QMP commands for internal snapshots, though.  Isn't the
> > > > > > biggest issue that savevm takes too much time to be a synchronous QMP
> > > > > > command?
> > > > > 
> > > > > Ultimately savevm/loadvm are using much of the migration code internally,
> > > > > but are not exposed as URI schemes. Could we perhaps take advantage of
> > > > > the internal common layer and define a migration URI scheme
> > > > > 
> > > > >    snapshot:<name>
> > > > > 
> > > > > where '<name>' is the name of the internal snapshot in the qcow2 file.
> > > > 
> > > > Let's include a node-name there, please. QEMU automagically deciding
> > > > where to store the VM state is one of the major problems of the HMP
> > > > interface.
> > > > 
> > > > And while we're at it, we can make it more future-proof by allowing to
> > > > specify arbitrary options:
> > > > 
> > > >     snapshot:node=<node-name>,name=<snapshot-name>
> > > > 
> > > > That would allow us to add something like compressed=on|off later.
> > > > Actually, compressed VM state sounds pretty nice. Why don't we have this
> > > > yet? The qcow2 format already provides everything you need for it.
> > > > 
> > > > > 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, you don't want to
> > > > waste space by keeping both the old and the current version of the page
> > > > content in the file.
> > > 
> > > The current snapshots are run with the CPU paused aren't they?  They
> > > share exactly the same RAM saving code as migration.
> > 
> > The commands block the monitor too, so I always assumed they were non-live
> 
> Yes, they are non-live, so RAM simply doesn't change and we that's the
> reason why we currently don't get duplicate pages in the snapshot.
> 
> But as I understand it, the whole point of migrate snapshot:... would be
> to make it live, so the potential duplication is something that we need
> to consider for it.

Yeah, true. Re-writing ram pages in-place could be doable if I added a
feature flag QIO_CHANNEL_FEATURE_SEEKABLE, so the migration code  can
tell if it will just append forever, or can seek back to rewrite.

Regards,
Daniel
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