From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([2001:4830:134:3::10]:50945)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <berrange@redhat.com>) id 1eZd2G-0006Kl-GQ
	for qemu-devel@nongnu.org; Thu, 11 Jan 2018 08:36:49 -0500
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <berrange@redhat.com>) id 1eZd2F-000797-8S
	for qemu-devel@nongnu.org; Thu, 11 Jan 2018 08:36:48 -0500
Date: Thu, 11 Jan 2018 13:36:35 +0000
From: "Daniel P. Berrange" <berrange@redhat.com>
Message-ID: <20180111133635.GH8326@redhat.com>
Reply-To: "Daniel P. Berrange" <berrange@redhat.com>
References: <20180107122336.29333-1-richiejp@f-m.fm>
	<fd5f5f12-e081-6f19-54af-3b560139bb78@redhat.com>
	<5cf19623-72ac-fb8b-2054-a60d42419ec6@redhat.com>
	<20180111130427.GG8326@redhat.com> <20180111132304.GD2669@work-vm>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <20180111132304.GD2669@work-vm>
Subject: Re: [Qemu-devel] [PATCH 1/2] Add save-snapshot,
 load-snapshot and delete-snapshot to QAPI
List-Id: <qemu-devel.nongnu.org>
List-Unsubscribe: <https://lists.nongnu.org/mailman/options/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel/>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <https://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: Max Reitz <mreitz@redhat.com>, Eric Blake <eblake@redhat.com>, Richard Palethorpe <richiejp@f-m.fm>, qemu-devel@nongnu.org, Qemu-block <qemu-block@nongnu.org>, quintela@redhat.com, armbru@redhat.com, rpalethorpe@suse.com

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.

> 
> > 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.

Regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|