Re: [Qemu-devel] [PATCH 1/2] Add save-snapshot, load-snapshot and delete-snapshot to QAPI

[Juan Quintela <quintela@redhat.com>]

"Daniel P. Berrange" <berrange@redhat.com> wrote:
> On Thu, Jan 11, 2018 at 01:23:05PM +0000, Dr. David Alan Gilbert wrote:
>> * Daniel P. Berrange (berrange@redhat.com) wrote:
>> > 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.
>> 
>> I had wondered about that; I'd just thought of doing the migration
>> saving to a block device rather than the rest of the snapshot
>> activity around it;
>> but I guess that's possible.
>
> One possible gotcha is whether the current savevm/loadvm QEMUFile impl
> actually does non-blocking I/O properly. eg same reason why we don't
> support a plain  file:<path> protocol - POSIX I/O on plain files doesn't
> honour O_NONBLOCK.  The block layer does AIO though, so we might be OK,
> depending on which block layer APIs the QEMUFile impl uses. I've not
> looked at the code recently though.

The blocking part is less important (for the write side), because we
have a thread there.  For loading .... it would be great to get one
migration thread also.

>> > 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.
>> 
>> What actually stops this working other than the sanity check in
>> migrate_incoming ?
>
> No idea really - not looked closely at the code implications.

It would be a plus for migration code, right now there are _two_
implementations, and savevm/loadvm one gets less love.

And we will check "much more" the way to load migration in a
non-pristine qemu, so ....

Later, Juan.