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Fri, 3 Jul 2020 17:10:03 +0000 (UTC) Date: Fri, 3 Jul 2020 18:10:00 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: "Dr. David Alan Gilbert" Subject: Re: [PATCH 2/6] migration: introduce savevm, loadvm, delvm QMP commands Message-ID: <20200703171000.GR2213227@redhat.com> References: <20200702175754.2211821-3-berrange@redhat.com> <20200702182452.GP1888119@redhat.com> <20200703154933.GE6641@work-vm> <20200703160235.GM2213227@redhat.com> <20200703161012.GH6641@work-vm> <20200703161654.GN2213227@redhat.com> <20200703162246.GI6641@work-vm> <20200703164958.GP2213227@redhat.com> <20200703170050.GK6641@work-vm> MIME-Version: 1.0 In-Reply-To: <20200703170050.GK6641@work-vm> User-Agent: Mutt/1.14.3 (2020-06-14) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=berrange@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Content-Disposition: inline Received-SPF: pass client-ip=207.211.31.120; envelope-from=berrange@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/07/03 02:53:25 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: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Cc: Kevin Wolf , Peter Krempa , qemu-block@nongnu.org, Juan Quintela , Markus Armbruster , qemu-devel@nongnu.org, Pavel Dovgalyuk , Paolo Bonzini , Max Reitz Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Fri, Jul 03, 2020 at 06:00:50PM +0100, Dr. David Alan Gilbert wrote: > * Daniel P. Berrangé (berrange@redhat.com) wrote: > > On Fri, Jul 03, 2020 at 05:22:46PM +0100, Dr. David Alan Gilbert wrote: > > > * Daniel P. Berrangé (berrange@redhat.com) wrote: > > > > On Fri, Jul 03, 2020 at 05:10:12PM +0100, Dr. David Alan Gilbert wrote: > > > > > * 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. > > > > > > > > The question is how much work that approach will be and whether anyone can > > > > realistically commit to doing that ? It looks like a much larger piece of > > > > work in both QEMU and libvirt side to do that. I don't want to see us stuck > > > > with a x-savevm for years because no one has resource to work on the perfect > > > > solution. If we did get a perfect solution at a point in future, we can > > > > still deprecate and then remove any "savevm" command we add to QMP. > > > > > > I'd at least like to understand that we've got a worklist for it though. > > > We've already got qemu_fopen_bdrv - what's actually wrong with that - is > > > that enough to do the migrate to a stream (given a tiny amount of > > > syntax) ? > > > > It is close. The migration code works with the QEMUFile layer, but in terms > > of the monitor commands the current framework expects a QIOChannel based > > QEMUFile. It would be possible to add new helpers to work with the bdrv > > backed QEMUFile. The ideal would be to create a QIOChannel impl that is > > backed by a block device though. At that point there would only be a single > > QEMUFile impl based on QIOChannel. > > > > That would be another step closer to unlocking the ability to eliminate the > > QEMUFile wrapper entirely. QEMUFile does I/O buffering too for performance, > > that could be done in a QIOChannel layer too, as that's a concept useful > > for other QIOChannel users too. > > There's some separate patches on list to do buffering in bdrv for > vmsave because apparently the qemufile ones don't play well with it. > > > That's still only the vmstate part though, and a solution is needed for > > the internal snapshot handling. > > Which bit is the bit that makes it block? The entire save_snapshot / load_snapshot methods really. They doing I/O operations throughout and this executes in context of the thread running the monitor, so their execution time is proportional to I/O time. 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 :|