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Date: Thu, 11 Jan 2018 13:04:27 +0000
From: "Daniel P. Berrange" <berrange@redhat.com>
Message-ID: <20180111130427.GG8326@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 1/2] Add save-snapshot,
 load-snapshot and delete-snapshot to QAPI
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To: Max Reitz <mreitz@redhat.com>
Cc: Eric Blake <eblake@redhat.com>, Richard Palethorpe <richiejp@f-m.fm>, qemu-devel@nongnu.org, Qemu-block <qemu-block@nongnu.org>, quintela@redhat.com, armbru@redhat.com, rpalethorpe@suse.com, dgilbert@redhat.com

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.

Then you could just use the regular migrate QMP commands for loading
and saving snapshots.  Might need a little extra work on the incoming
side, since we need to be able to load snapshots, despite QEMU not
being started with '-incoming defer', but might still be doable ?
This would theoretically give us progress monitoring, cancellation,
etc for free.



Regards,
Daniel
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