Re: [PATCH 2/6] migration: introduce savevm, loadvm, delvm QMP commands

From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
To: Peter Krempa <pkrempa@redhat.com>
Cc: "Kevin Wolf" <kwolf@redhat.com>,
	"Daniel P. Berrangé" <berrange@redhat.com>,
	qemu-block@nongnu.org, "Juan Quintela" <quintela@redhat.com>,
	"Markus Armbruster" <armbru@redhat.com>,
	qemu-devel@nongnu.org, "Max Reitz" <mreitz@redhat.com>,
	"Pavel Dovgalyuk" <pavel.dovgaluk@ispras.ru>,
	"Paolo Bonzini" <pbonzini@redhat.com>
Subject: Re: [PATCH 2/6] migration: introduce savevm, loadvm, delvm QMP commands
Date: Fri, 3 Jul 2020 17:26:30 +0100	[thread overview]
Message-ID: <20200703162630.GJ6641@work-vm> (raw)
In-Reply-To: <20200703162417.GA3255981@angien.pipo.sk>

* Peter Krempa (pkrempa@redhat.com) wrote:
> On Fri, Jul 03, 2020 at 17:10:12 +0100, Dr. David Alan Gilbert wrote:
> > * Daniel P. BerrangÃ© (berrange@redhat.com) wrote:
> > > On Fri, Jul 03, 2020 at 04:49:33PM +0100, Dr. David Alan Gilbert wrote:
> > > > * Daniel P. BerrangÃ© (berrange@redhat.com) wrote:
> > > > > On Thu, Jul 02, 2020 at 01:12:52PM -0500, Eric Blake wrote:
> > > > > > On 7/2/20 12:57 PM, Daniel P. BerrangÃ© wrote:
> 
> [...]
> 
> > > > Remind me, what was the problem with just making a block: migration
> > > > channel, and then we can migrate to it?
> > > 
> > > migration only does vmstate, not disks. The current blockdev commands
> > > are all related to external snapshots, nothing for internal snapshots
> > > AFAIK. So we still need commands to load/save internal snapshots of
> > > the disk data in the qcow2 files.
> > > 
> > > So we could look at loadvm/savevm conceptually as a syntax sugar above
> > > a migration transport that targets disk images, and blockdev QMP command
> > > that can do internal snapshots. Neither of these exist though and feel
> > > like a significantly larger amount of work than using existing functionality
> > > that is currently working.
> > 
> > I think that's what we should aim for; adding this wrapper isn't gaining
> > that much without moving a bit towards that; so I would stick with the
> > x- for now.
> 
> Relying on the HMP variants is IMO even worse. Error handling is
> terrible there. I'd vote even for a straight wrapper without any logic
> at this point. IMO it's just necessary to document that it's an
> intermediate solution which WILL be deprecated and removed as soon as a
> suitable replacement is in place.
> 
> Not doing anything is the argument we hear for multiple years now and
> savevm/delvm/loadvm are now the only 3 commands used via the HMP wrapper
> in libvirt.
> 
> Since deprecation is now a thing I think we can add a disposable
> inteface. In the end HMP will or will not need to remain anyways and the
> deprecation there is IMO less clear.

Only if we come up with a list of what we actually need to do to
properly fix it; I'm not suggesting we actually need to do the work, but
we should figure out what we need to do.

Dave

--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK