Re: [PATCH 5/6] migration: support excluding block devs in QMP snapshot commands

[Daniel P . Berrangé]

On Mon, Jul 06, 2020 at 05:57:08PM +0200, Kevin Wolf wrote:
> Am 02.07.2020 um 19:57 hat Daniel P. Berrangé geschrieben:
> > This wires up support for a new "exclude" parameter to the QMP commands
> > for snapshots (savevm, loadvm, delvm). This parameter accepts a list of
> > block driver state node names.
> > 
> > One use case for this would be a VM using OVMF firmware where the
> > variables store is a raw disk image. Ideally the variable store would be
> > qcow2, allowing its contents to be included in the snapshot, but
> > typically today the variable store is raw. It is still useful to be able
> > to snapshot VMs using OVMF, even if the varstore is excluded, as the
> > main OS disk content is usually the stuff the user cares about.
> > 
> > Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
> 
> Wouldn't it be better to take an optional list of nodes to _include_
> that just defaults to our current set of nodes?
> 
> The problem with excluding is that we already don't snapshot many nodes,
> and the criteria to choose the right ones weren't entirely obvious, so
> we just went with something that seemed to make the most sense. But the
> management application may actually want to snapshot more nodes than we
> cover by default.
> 
> I feel things become clearer and less surprising if the client just
> tells us explicitly which images are supposed to be snapshotted instead
> of adding exceptions on top of a default selection that we're already
> not really sure about.

I thought that QEMU just excluded nodes which are not capable of being
snapshotted. By using exclusions, the mgmt apps don't have to know
about what types of storage driver QEMU supports snapshots on, just let
QEMU decide. It also felt simpler to use exclusions as normal case would
be to snapshot everything.   Both inclusions/exclusions are easy to
implement in QEMU though.

Regards,
Daniel
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