[libvirt] [PATCH 5/4] qemu: properly revert to offline snapshots

[Philipp Hahn]

Hello Eric,

thank you for the long description of <active>

Am Mittwoch 10 August 2011 14:18:22 schrieb Eric Blake:
> Right now, qemu's _only_ use of <active> (which is set to non-zero by
> qemuDomainSnapshotSetCurrentActive) is to tell qemuBuildCommandLine to
> use -loadvm to revert to that point, rather than to do a fresh boot.
> Your approach of keeping the call means that qemuBuildCommandLine has to
> special-case to not use -loadvm for inactive snapshots; whereas my
> approach of no longer marking the snapshot active for inactive snapshots
> means that the existing qemuBuildCommandLine no longer sees an active
> snapshot in the first place; both approaches end up omitting the -loadvm
> call on the next domain boot, but mine does it with less code and
> without using <active>.

Since <active> is persisted in the snapshot/ directory, I didn't want to 
change that, so I didn't remove that call.

> Once that is done, qemu is no longer using active for its own purposes,
> so conf/domain_conf can then take over <active> to uniformly identify
> which snapshot is current as part of the snapshot xml (that is, all but
> one snapshot will be <active>0</active>, and the current snapshot will
> be <active>1</active>).  Every time the current snapshot changes, ...

I find the name "current" highly confusing, because at least for Qemus 
internal shapshot it is something more like "last snapshot restored": As soon 
as the VM begins to run, it diverges from the snapshot state and is no longer 
equal to it, so it is not "current" any more. Only when restoring offline or 
paused states would I have the chance to realistically see the VM being in 
the same state as the snapshot.
t branch.

So if <active> is only used to track the last restored snapshot and to fill in 
the <parent> data in the <domsinsnapshot> data, your patch looks fine.

Sincerely
Philipp Hahn
-- 
Philipp Hahn           Open Source Software Engineer      hahn at univention.de
Univention GmbH        Linux for Your Business        fon: +49 421 22 232- 0
Mary-Somerville-Str.1  D-28359 Bremen                 fax: +49 421 22 232-99
                                                   http://www.univention.de/
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