On Thu, Sep 18, 2025 at 15:45:21 +0100, Daniel P. Berrangé wrote: > On Thu, Sep 18, 2025 at 04:16:56PM +0200, Juraj Marcin wrote: > > If there is no outgoing traffic from the destination side (this can be > > caused for example by a workload with no page faults or paused machine), > > QEMU has no way of knowing if the connection is still working or not. > > The TCP stack doesn't treat no incoming traffic as a sign of a broken > > connection. Therefore, QEMU would stay in postcopy-active waiting for > > pages indefinitely. > > > > Also, libvirt might not be aware of a connection dropout between QEMUs, > > if libvirt's connection is intact, especially if libvirt daemons are > > communicating through some central entity that is managing the migration > > and not directly. And to do postcopy migration recovery, libvirt needs > > both sides to be in postcopy-paused state. > > Whether keepalive timeouts are at the QEMU level or global kernel > level, there will always be situations where the timeouts are too > long. Apps/admins can have out of band liveliness checks between > hosts that detect a problem before the keepalives will trigger > and shouldn't have to wait to recover migration, once they have > resolved the underlying network issue. > > There needs to be a way to initiate post-copy recovery regardless > of whether we've hit a keepalive timeout. Especially if we can > see one QEMU in postcopy-paused, but not the other side, it > doesn't appear to make sense to block the recovery process. > > The virDomainJobCancel command can do a migrate-cancel on the > src, but it didn't look like we could do the same on the dst. > Unless I've overlooked something, Libvirt needs to gain a way > to explicitly force both sides into the postcopy-paused state, > and thus be able to immediately initiate recovery. > > > Alternatively, there also might be an issue with the connection between > > libvirt daemons, but not the migration connection. Even if the libvirt > > connection fails, the migration is not paused, rather libvirt lets the > > migration finish normally. Similarly, if the libvirt connection is > > broken up due to, for example, libvirt daemon restart, the ongoing > > migration is not paused, but after the libvirt daemon starts again, it > > sees an ongoing migration and lets it finish. > > Whole this is a reliability issue for libvirt, this doesn't have > any bearing on migration keepalive timeouts, as we're only concerned > about QEMU connections. > > > Additionally, libvirt uses its own internal keep-alive packets with much > > more aggressive timeouts, waiting 5 - 10 seconds idle before sending a > > keep-alive packet and then killing the connection if there is no > > response in 30 seconds. > > Yep, this keepalive is very aggressive and has frequently caused > problems with libvirt connections being torn down inappropriately.
This was happening when a link was saturated with storage migrations and the keepalive messages in a separate (and otherwise idle) connection were not sent in time. We haven't seen any reports for quite some time. I believe it was identified as a kernel bug a few years ago and then the reports stopped. Jirka
