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[99.254.121.117]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-6bf6fdd9c68sm18637636d6.3.2024.08.16.08.34.11 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 16 Aug 2024 08:34:12 -0700 (PDT) Date: Fri, 16 Aug 2024 11:34:10 -0400 From: Peter Xu To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Cc: Steven Sistare , qemu-devel@nongnu.org, Fabiano Rosas , David Hildenbrand , Marcel Apfelbaum , Eduardo Habkost , Philippe Mathieu-Daude , Paolo Bonzini , Markus Armbruster Subject: Re: [PATCH V2 00/11] Live update: cpr-exec Message-ID: References: <1719776434-435013-1-git-send-email-steven.sistare@oracle.com> <5776f6b4-c55f-4e56-aac5-bee7e441ad15@oracle.com> <46b63356-9602-4fa2-9d31-186f5f85151f@oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Received-SPF: pass client-ip=170.10.133.124; envelope-from=peterx@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -21 X-Spam_score: -2.2 X-Spam_bar: -- X-Spam_report: (-2.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.131, 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_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Fri, Aug 16, 2024 at 04:16:50PM +0100, Daniel P. Berrangé wrote: > On Fri, Aug 16, 2024 at 11:06:10AM -0400, Peter Xu wrote: > > On Thu, Aug 15, 2024 at 04:55:20PM -0400, Steven Sistare wrote: > > > On 8/13/2024 3:46 PM, Peter Xu wrote: > > > > On Tue, Aug 06, 2024 at 04:56:18PM -0400, Steven Sistare wrote: > > > > > > The flipside, however, is that localhost migration via 2 separate QEMU > > > > > > processes has issues where both QEMUs want to be opening the very same > > > > > > file, and only 1 of them can ever have them open. > > > > > > > > I thought we used to have similar issue with block devices, but I assume > > > > it's solved for years (and whoever owns it will take proper file lock, > > > > IIRC, and QEMU migration should properly serialize the time window on who's > > > > going to take the file lock). > > > > > > > > Maybe this is about something else? > > > > > > I don't have an example where this fails. > > > > > > I can cause "Failed to get "write" lock" errors if two qemu instances open > > > the same block device, but the error is suppressed if you add the -incoming > > > argument, due to this code: > > > > > > blk_attach_dev() > > > if (runstate_check(RUN_STATE_INMIGRATE)) > > > blk->disable_perm = true; > > > > Yep, this one is pretty much expected. > > > > > > > > > > Indeed, and "files" includes unix domain sockets. > > > > > > More on this -- the second qemu to bind a unix domain socket for listening > > > wins, and the first qemu loses it (because second qemu unlinks and recreates > > > the socket path before binding on the assumption that it is stale). > > > > > > One must use a different name for the socket for second qemu, and clients > > > that wish to connect must be aware of the new port. > > > > > > > > Network ports also conflict. > > > > > cpr-exec avoids such problems, and is one of the advantages of the method that > > > > > I forgot to promote. > > > > > > > > I was thinking that's fine, as the host ports should be the backend of the > > > > VM ports only anyway so they don't need to be identical on both sides? > > > > > > > > IOW, my understanding is it's the guest IP/ports/... which should still be > > > > stable across migrations, where the host ports can be different as long as > > > > the host ports can forward guest port messages correctly? > > > > > > Yes, one must use a different host port number for the second qemu, and clients > > > that wish to connect must be aware of the new port. > > > > > > That is my point -- cpr-transfer requires fiddling with such things. > > > cpr-exec does not. > > > > Right, and my understanding is all these facilities are already there, so > > no new code should be needed on reconnect issues if to support cpr-transfer > > in Libvirt or similar management layers that supports migrations. > > Note Libvirt explicitly blocks localhost migration today because > solving all these clashing resource problems is a huge can of worms > and it can't be made invisible to the user of libvirt in any practical > way. Ahhh, OK. I'm pretty surprised by this, as I thought at least kubevirt supported local migration somehow on top of libvirt. Does it mean that cpr-transfer is a no-go in this case at least for Libvirt to consume it (as cpr-* is only for local host migrations so far)? Even if all the rest issues we're discussing with cpr-exec, is that the only way to go for Libvirt, then? -- Peter Xu