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envelope-from=peterx@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, 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_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 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 Wed, Sep 10, 2025 at 08:10:57AM +0100, Daniel P. Berrangé wrote: > On Tue, Sep 09, 2025 at 05:58:49PM -0400, Peter Xu wrote: > > On Tue, Sep 09, 2025 at 04:09:23PM +0100, Daniel P. Berrangé wrote: > > > On Tue, Sep 09, 2025 at 05:01:24PM +0200, Juraj Marcin wrote: > > > > From: Juraj Marcin > > > > > > > > Usual system defaults for TCP keep-alive options are too long for > > > > migration workload. On Linux, a TCP connection waits idle for 2 hours > > > > before it starts checking if the connection is not broken. > > > > > > > > Now when InetSocketAddress supports keep-alive options [1], this patch > > > > applies migration specific defaults if they are not supplied by the user > > > > or the management software. With these defaults, a migration TCP stream > > > > waits idle for 1 minute and then sends 5 TCP keep-alive packets in 30 > > > > second interval before considering the connection as broken. > > > > > > > > System defaults can be still used by explicitly setting these parameters > > > > to 0. > > > > > > IMHO this is not a good idea. This is a very short default, which > > > may be fine for the scenario where your network conn is permanently > > > dead, but it is going to cause undesirable failures when the network > > > conn is only temporarily dead. > > > > > > Optimizing defaults for temporary outages is much more preferrable > > > as that maximises reliability of migration. In the case of permanent > > > outages, it is already possible to tear down the connection without > > > waiting for a keep-alive timeout, and liveliness checks can also be > > > perform by the mgmt app at a higher level too. The TCP keepalives > > > are just an eventual failsafe, and having those work on a long > > > timeframe is OK. > > > > For precopy it looks fine indeed, because migrate_cancel should always work > > on src if src socket hanged, and even if dest QEMU socket hanged, it can > > simply be killed if src QEMU can be gracefully cancelled and rolled back to > > RUNNING, disregarding the socket status on dest QEMU. > > > > For postcopy, we could still use migrate_pause to enforce src shutdown(). > > Initially I thought we have no way of doing that for dest QEMU, but I just > > noticed two years ago I added that to dest QEMU for migrate_paused when > > working on commit f8c543e808f20b.. So looks like that part is covered too, > > so that if dest QEMU socket hanged we can also kick it out. > > > > I'm not 100% sure though, on whether shutdown() would always be able to > > successfully kick out the hanged socket while the keepalive is ticking. Is > > it guaranteed? > > I don't know about shutdown(), but close() certainly works. If shutdown() > is not sufficient, then IMHO the migration code would need the ability to > use close() to deal with this situation. > > > > I also am not sure if that happens, whether libvirt would automatically do > > that, or provide some way so the user can trigger that. The goal IIUC here > > is we shouldn't put user into a situation where the migration hanged but > > without any way to either cancel or recover. With the default values Juraj > > provided here, it makes sure the hang won't happen more than a few minutes, > > which sounds like a sane timeout value. > > Sufficient migration QMP commands should exist to ensure migration can > always be cancelled. Short keepalive timeouts should not be considered > a solution to any gaps in that respect. > > Also there is yank, but IMHO apps shouldn't have to rely on yank - I see > yank as a safety net for apps to workaround limitations in QEMU. The QMP facility looks to be all present, which is migrate-cancel and migrate-pause mentioned above. For migrate_cancel (of precopy), is that Ctrl-C of "virsh migrate"? Does libvirt exposes migrate_pause via any virsh command? IIUC that's the only official way of pausing a postcopy VM on either side. I also agree we shouldn't make yank the official tool to use. OTOH, the default timeouts work without changing libvirt, making sure the customers will not be stuck in a likely-failing network for hours without providing a way to properly detach and recover when it's wanted. Thanks, -- Peter Xu