On Fri, Sep 12, 2025 at 11:02:12AM -0400, Peter Xu wrote: > On Fri, Sep 12, 2025 at 11:20:01AM +0100, Daniel P. Berrangé wrote: > > On Wed, Sep 10, 2025 at 12:36:44PM -0400, Peter Xu wrote: > > > 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 <[email protected]> > > > > > > > > > > > > > > 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. > > > > virsh will call virDomainAbortJob when Ctrl-C is done to a 'migrate' > > command. > > > > virDomainAbortJob will call migrate-cancel for pre-copy, or > > 'migrate-pause' for post-copy. > > Would it call "migrate-pause" on both sides?
Not 100% sure, but with virDomainAbortJob I think libvirt only calls migrate-pause on the source host. > I believe the problem we hit was, when during postcopy and the NIC was > misfunctioning, src fell into postcopy-paused successfully but dest didn't, > stuck in postcopy-active. If something has interrupted src<->dst host comms for QEMU it may well impact libvirt <-> libvirt comms too, unless migration was being done over a separate NIC than the mgmt LAN. IOW, it may be impossible for libvirt to call migrate-pause on both sides, at least not until the NIC problem has been resolved. > We'll want to make sure both sides to be kicked into paused stage to > recover. Otherwise dest can hang in the stage for hours until the watchdog > timeout triggers. Once the network problem has been resolved, then it ought to be possible to get libvirt to issue 'migrate-pause' on both hosts, and thus be able to recover. Possibly the act of starting migration recovery in libvirt should attempt to issue 'migrate-pause' to cleanup the previously running migration if it is still in the stuck state. > > > > > > > > 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. > > > > "timeouts work" has the implicit assumpton that the only reason a > > timeout will fire is due to a unrecoverable situation. IMHO that > > assumption is not valid. > > I agree adjusting timeout is not the best. > > If we can have solid way to kick two sides out, I think indeed we don't > need to change the timeout. > > If not, we may still need to provide a way to allow user to try continue > when the user found that the network is behaving abnormal. > > Here adjusting timeout is slightly better than any adhoc socket timeout > that we'll adjust: it's the migration timeout, and we only have two cases: > (1) precopy, which is ok to fail and retried, (2) postcopy, which is also > ok to fail and recovered. Fail & retry/recover is not without cost / risk though. Users can have successful migrations that are many hours long when dealing with big VMs. IOW, returning to the start of pre-copy could be a non-trivial time delay. Consider if the reason for the migration is to evacuate workloads off a host that is suffering technical problems. It could well be that periodic unexpected network outages are what is triggering the need to evacuate workloads. If we timeout a migration with keepalives they may never be able to get through a migration op quickly enough, or they can be delayed such that the host has a fatal error loosing the workload before the retried migration is complete. IMHO, once a migration has been started we should not proactively interrupt that with things like keepalives, unless the admin made a concious decision they wanted that behaviour enabled. With 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 :|
