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

[Richard Palethorpe]

Hello,

Juan Quintela writes:

> "Daniel P. Berrange" <address@hidden> wrote:
>> On Thu, Jan 11, 2018 at 01:23:05PM +0000, Dr. David Alan Gilbert wrote:
>>> * Daniel P. Berrange (address@hidden) 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.

This looks like the best option so far for my use case.

-- 
Thank you,
Richard.