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

[Peter Krempa]

On Fri, Jul 03, 2020 at 17:10:12 +0100, Dr. David Alan Gilbert wrote:
> * Daniel P. Berrangé (berrange@redhat.com) wrote:
> > On Fri, Jul 03, 2020 at 04:49:33PM +0100, Dr. David Alan Gilbert wrote:
> > > * Daniel P. Berrangé (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. Berrangé 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.