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 <jmar...@redhat.com> > > > > > > > > > > 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. > 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. 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 :|
