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

[Dr. David Alan Gilbert]

* 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:
> > > > > savevm, loadvm and delvm are some of the few commands that have never
> > > > > been converted to use QMP. The primary reason for this lack of
> > > > > conversion is that they block execution of the thread for as long as
> > > > > they run.
> > > > > 
> > > > > Despite this downside, however, libvirt and applications using libvirt
> > > > > has used these commands for as long as QMP has existed, via the
> > > > > "human-monitor-command" passthrough command. IOW, while it is clearly
> > > > > desirable to be able to fix the blocking problem, this is not an
> > > > > immediate obstacle to real world usage.
> > > > > 
> > > > > Meanwhile there is a need for other features which involve adding new
> > > > > parameters to the commands. This is possible with HMP passthrough, but
> > > > > it provides no reliable way for apps to introspect features, so using
> > > > > QAPI modelling is highly desirable.
> > > > > 
> > > > > This patch thus introduces trival savevm, loadvm, delvm commands
> > > > 
> > > > trivial
> > > > 
> > > > > to QMP that are functionally identical to the HMP counterpart, 
> > > > > including
> > > > > the blocking problem.
> > > > 
> > > > Should we name them 'x-savevm', 'x-loadvm', 'x-delvm' to give ourselves 
> > > > room
> > > > to change them when we DO solve the blocking issue?  Or will the 
> > > > solution of
> > > > the blocking issue introduce new QMP commands, at which point we can 
> > > > add QMP
> > > > deprecation markers on these commands to eventually retire them?
> > > 
> > > I was in two minds about this, so I'm open to arguments either way.
> > > 
> > > The primary goal is for libvirt to consume the APIs as soon as possible,
> > > and generally libvirt doesn't want todo this is they are declared 
> > > experimental
> > > via a "x-" prefix. So that pushes me away from "x-".
> > > 
> > > If we don't have an "x-" prefix and want to make changes, we can add extra
> > > parameters to trigger new behaviour in backwards compatible manner. Or we 
> > > can
> > > simply deprecate these commands, deleting them 2 releases later, while 
> > > adding
> > > completely new commands.
> > > 
> > > If we think the prposed design will definitely need incompatible changes 
> > > in
> > > a very short time frame though, that would push towards "x-".
> > > 
> > > So IMHO the right answer largely depends on whether there is a credible
> > > strategy to implement the ideal non-blocking solution in a reasonable 
> > > amount
> > > of time. I can't justify spending much time on this myself, but I'm 
> > > willing
> > > to consider & try proposals for solving the blocking problem if they're 
> > > not
> > > too complex / invasive.
> > 
> > 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.

Dave

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