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Tue, 7 Jul 2020 06:38:56 +0000 (UTC) Date: Tue, 7 Jul 2020 08:38:53 +0200 From: Peter Krempa To: Kevin Wolf Subject: Re: [PATCH 2/6] migration: introduce savevm, loadvm, delvm QMP commands Message-ID: <20200707063853.GA9048@angien.pipo.sk> 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> <20200706161555.GE5052@linux.fritz.box> MIME-Version: 1.0 In-Reply-To: <20200706161555.GE5052@linux.fritz.box> X-PGP-Key-ID: 0xD018682B X-PGP-Key-Fingerprint: D294 FF38 A6A2 BF40 6C75 5DEF 36EC 16AC D018 682B User-Agent: Mutt/1.14.3 (2020-06-14) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=pkrempa@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=pkrempa@redhat.com; helo=us-smtp-delivery-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/07/07 00:20:54 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 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: Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= , qemu-block@nongnu.org, Juan Quintela , qemu-devel@nongnu.org, Markus Armbruster , "Dr. David Alan Gilbert" , Max Reitz , Pavel Dovgalyuk , Paolo Bonzini Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Mon, Jul 06, 2020 at 18:15:55 +0200, Kevin Wolf wrote: > Am 03.07.2020 um 18:02 hat Daniel P. Berrangé geschrieben: > > 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: [...] > > 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. > > There is blockdev-snapshot-internal-sync, which does a disk-only > snapshot of a single node. A snapshot of multiple nodes can be taken by > putting multiple blockdev-snapshot-internal-sync inside a 'transaction' > command. Libvirt never implemented support for disk-only internal snapshots as I didn't think they are worth it. We also made a mistake by using the VIR_DOMAIN_SNAPSHOT_DISK_ONLY to switch to an external snapshot, so while the XML can modify the snapshot to be internal it's not very clear nor user-friendly to force an internal disk only snapshot. > If we want to build on top of this, we'd have to implement a > transactionable command for storing only the VM state to a specific > node. This would probably still be a long-running job. IMO we really want this also for external snapshots. Driving the migration as standard migration is really suboptimal especially when the user wants minimal downtime. Transactioning a post-copy style copy-on write migration would simplify this a lot. I agree though that this is for a different conversation.