Re: [Qemu-devel] [PATCH 3/3] qapi: Introduce blockdev-query-group-snapshot-failure

[Eric Blake <eblake@redhat.com>]

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On 02/20/2012 10:31 AM, Jeff Cody wrote:
> In the case of a failure in a group snapshot, it is possible for
> multiple file image failures to occur - for instance, failure of
> an original snapshot, and then failure of one or more of the
> attempted reopens of the original.
> 
> Knowing all of the file images which failed could be useful or
> critical information, so this command returns a list of strings
> containing the filenames of all failures from the last
> invocation of blockdev-group-snapshot-sync.

Meta-question:

Suppose that the guest is running when we issue
blockdev-group-snapshot-sync - in that case, qemu is responsible for
pausing and then resuming the guest.  On success, this makes sense.  But
what happens on failure?

If we only fail at creating one snapshot, but successfully roll back the
rest of the set, should the guest be resumed (as if the command had
never been attempted), or should the guest be left paused?

On the other hand, if we fail at creating one snapshot, as well as fail
at rolling back, then that argues that we _cannot_ resume the guest,
because we no longer have a block device open.

This policy needs to be documented in one (or both) of the two new
monitor commands, and we probably ought to make sure that if the guest
is left paused where it had originally started as running, then an
appropriate event is also emitted.

For blockdev-snapshot-sync, libvirt was always pausing qemu before
issuing the snapshot, then resuming afterwards; but now that we have the
ability to make the set atomic, I'm debating about whether libvirt still
needs to pause qemu, or whether it can now rely on qemu doing the right
things about pausing and resuming as part of the snapshot command.

-- 
Eric Blake   eblake@redhat.com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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