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Fri, 3 Jul 2020 16:10:14 +0000 (UTC) Date: Fri, 3 Jul 2020 17:10:12 +0100 From: "Dr. David Alan Gilbert" To: Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= Subject: Re: [PATCH 2/6] migration: introduce savevm, loadvm, delvm QMP commands Message-ID: <20200703161012.GH6641@work-vm> References: <20200702175754.2211821-1-berrange@redhat.com> <20200702175754.2211821-3-berrange@redhat.com> <20200702182452.GP1888119@redhat.com> <20200703154933.GE6641@work-vm> <20200703160235.GM2213227@redhat.com> MIME-Version: 1.0 In-Reply-To: <20200703160235.GM2213227@redhat.com> User-Agent: Mutt/1.14.5 (2020-06-23) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=dgilbert@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: 8bit Content-Disposition: inline Received-SPF: pass client-ip=207.211.31.81; envelope-from=dgilbert@redhat.com; helo=us-smtp-delivery-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/07/03 01:34:15 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -30 X-Spam_score: -3.1 X-Spam_bar: --- X-Spam_report: (-3.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=-0.01, RCVD_IN_MSPIKE_WL=-0.01, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, URIBL_BLOCKED=0.001 autolearn=_AUTOLEARN X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Kevin Wolf , Peter Krempa , qemu-block@nongnu.org, Juan Quintela , qemu-devel@nongnu.org, Markus Armbruster , Pavel Dovgalyuk , Paolo Bonzini , Max Reitz Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" * Daniel P. Berrangé (berrange@redhat.com) wrote: > On Fri, Jul 03, 2020 at 04:49:33PM +0100, Dr. David Alan Gilbert wrote: > > * Daniel P. Berrangé (berrange@redhat.com) wrote: > > > On Thu, Jul 02, 2020 at 01:12:52PM -0500, Eric Blake wrote: > > > > On 7/2/20 12:57 PM, Daniel P. Berrangé wrote: > > > > > savevm, loadvm and delvm are some of the few commands that have never > > > > > been converted to use QMP. The primary reason for this lack of > > > > > conversion is that they block execution of the thread for as long as > > > > > they run. > > > > > > > > > > Despite this downside, however, libvirt and applications using libvirt > > > > > has used these commands for as long as QMP has existed, via the > > > > > "human-monitor-command" passthrough command. IOW, while it is clearly > > > > > desirable to be able to fix the blocking problem, this is not an > > > > > immediate obstacle to real world usage. > > > > > > > > > > Meanwhile there is a need for other features which involve adding new > > > > > parameters to the commands. This is possible with HMP passthrough, but > > > > > it provides no reliable way for apps to introspect features, so using > > > > > QAPI modelling is highly desirable. > > > > > > > > > > This patch thus introduces trival savevm, loadvm, delvm commands > > > > > > > > trivial > > > > > > > > > to QMP that are functionally identical to the HMP counterpart, including > > > > > the blocking problem. > > > > > > > > Should we name them 'x-savevm', 'x-loadvm', 'x-delvm' to give ourselves room > > > > to change them when we DO solve the blocking issue? Or will the solution of > > > > the blocking issue introduce new QMP commands, at which point we can add QMP > > > > deprecation markers on these commands to eventually retire them? > > > > > > I was in two minds about this, so I'm open to arguments either way. > > > > > > The primary goal is for libvirt to consume the APIs as soon as possible, > > > and generally libvirt doesn't want todo this is they are declared experimental > > > via a "x-" prefix. So that pushes me away from "x-". > > > > > > If we don't have an "x-" prefix and want to make changes, we can add extra > > > parameters to trigger new behaviour in backwards compatible manner. Or we can > > > simply deprecate these commands, deleting them 2 releases later, while adding > > > completely new commands. > > > > > > If we think the prposed design will definitely need incompatible changes in > > > a very short time frame though, that would push towards "x-". > > > > > > So IMHO the right answer largely depends on whether there is a credible > > > strategy to implement the ideal non-blocking solution in a reasonable amount > > > of time. I can't justify spending much time on this myself, but I'm willing > > > to consider & try proposals for solving the blocking problem if they're not > > > too complex / invasive. > > > > Remind me, what was the problem with just making a block: migration > > channel, and then we can migrate to it? > > migration only does vmstate, not disks. The current blockdev commands > are all related to external snapshots, nothing for internal snapshots > AFAIK. So we still need commands to load/save internal snapshots of > the disk data in the qcow2 files. > > So we could look at loadvm/savevm conceptually as a syntax sugar above > a migration transport that targets disk images, and blockdev QMP command > that can do internal snapshots. Neither of these exist though and feel > like a significantly larger amount of work than using existing functionality > that is currently working. I think that's what we should aim for; adding this wrapper isn't gaining that much without moving a bit towards that; so I would stick with the x- for now. Dave > 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 :| -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK