* Felipe Franciosi (fel...@nutanix.com) wrote: > > > > On Sep 30, 2019, at 6:11 PM, Dr. David Alan Gilbert <dgilb...@redhat.com> > > wrote: > > > > * Felipe Franciosi (fel...@nutanix.com) wrote: > >> > >> > >>> On Sep 30, 2019, at 5:03 PM, Dr. David Alan Gilbert <dgilb...@redhat.com> > >>> wrote: > >>> > >>> * Felipe Franciosi (fel...@nutanix.com) wrote: > >>>> Hi David, > >>>> > >>>>> On Sep 30, 2019, at 3:29 PM, Dr. David Alan Gilbert > >>>>> <dgilb...@redhat.com> wrote: > >>>>> > >>>>> * Felipe Franciosi (fel...@nutanix.com) wrote: > >>>>>> Heyall, > >>>>>> > >>>>>> We have a use case where a host should self-fence (and all VMs should > >>>>>> die) if it doesn't hear back from a heartbeat within a certain time > >>>>>> period. Lots of ideas were floated around where libvirt could take > >>>>>> care of killing VMs or a separate service could do it. The concern > >>>>>> with those is that various failures could lead to _those_ services > >>>>>> being unavailable and the fencing wouldn't be enforced as it should. > >>>>>> > >>>>>> Ultimately, it feels like Qemu should be responsible for this > >>>>>> heartbeat and exit (or execute a custom callback) on timeout. > >>>>> > >>>>> It doesn't feel doing it inside qemu would be any safer; something > >>>>> outside QEMU can forcibly emit a kill -9 and qemu *will* stop. > >>>> > >>>> The argument above is that we would have to rely on this external > >>>> service being functional. Consider the case where the host is > >>>> dysfunctional, with this service perhaps crashed and a corrupt > >>>> filesystem preventing it from restarting. The VMs would never die. > >>> > >>> Yeh that could fail. > >>> > >>>> It feels like a Qemu timer-driven heartbeat check and calls abort() / > >>>> exit() would be more reliable. Thoughts? > >>> > >>> OK, yes; perhaps using a timer_create and telling it to send a fatal > >>> signal is pretty solid; it would take the kernel to do that once it's > >>> set. > >> > >> I'm confused about why the kernel needs to be involved. If this is a > >> timer off the Qemu main loop, it can just check on the heartbeat > >> condition (which should be customisable) and call abort() if that's > >> not satisfied. If you agree on that I'd like to talk about how that > >> check could be made customisable. > > > > There are times when the main loop can get blocked even though the CPU > > threads can be running and can in some configurations perform IO > > even without the main loop (I think!). > > Ah, that's a very good point. Indeed, you can perform IO in those > cases specially when using vhost devices. > > > By setting a timer in the kernel that sends a signal to qemu, the kernel > > will send that signal however broken qemu is. > > Got you now. That's probably better. Do you reckon a signal is > preferable over SIGEV_THREAD?
Not sure; probably the safest is getting the kernel to SIGKILL it - but that's a complete nightmare to debug - your process just goes *pop* with no apparent reason why. I've not used SIGEV_THREAD - it looks promising though. > I'm still wondering how to make this customisable so that different > types of heartbeat could be implemented (preferably without creating > external dependencies per discussion above). Thoughts welcome. Yes, you need something to enable it, and some safe way to retrigger the timer. A qmp command marked as 'oob' might be the right way - another qm command can't block it. Dave > F. > > > > >> > >>> > >>> IMHO the safer way is to kick the host off the network by reprogramming > >>> switches; so even if the qemu is actually alive it can't get anywhere. > >>> > >>> Dave > >> > >> Naturally some off-host STONITH is preferable, but that's not always > >> available. A self-fencing mechanism right at the heart of the emulator > >> can do the job without external hardware dependencies. > > > > Dave > > > >> Cheers, > >> Felipe > >> > >>> > >>> > >>>> Felipe > >>>> > >>>>> > >>>>>> Does something already exist for this purpose which could be used? > >>>>>> Would a generic Qemu-fencing infrastructure be something of interest? > >>>>> Dave > >>>>> > >>>>> > >>>>>> Cheers, > >>>>>> F. > >>>>>> > >>>>> -- > >>>>> Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK > >>>> > >>> -- > >>> Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK > >> > > -- > > Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK > -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK