Re: Thoughts on VM fence infrastructure

[Dr. David Alan Gilbert]

* Felipe Franciosi (address@hidden) wrote:
> 
> 
> > On Sep 30, 2019, at 6:11 PM, Dr. David Alan Gilbert <address@hidden> wrote:
> > 
> > * Felipe Franciosi (address@hidden) wrote:
> >> 
> >> 
> >>> On Sep 30, 2019, at 5:03 PM, Dr. David Alan Gilbert <address@hidden> 
> >>> wrote:
> >>> 
> >>> * Felipe Franciosi (address@hidden) wrote:
> >>>> Hi David,
> >>>> 
> >>>>> On Sep 30, 2019, at 3:29 PM, Dr. David Alan Gilbert <address@hidden> 
> >>>>> wrote:
> >>>>> 
> >>>>> * Felipe Franciosi (address@hidden) 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 / address@hidden / Manchester, UK
> >>>> 
> >>> --
> >>> Dr. David Alan Gilbert / address@hidden / Manchester, UK
> >> 
> > --
> > Dr. David Alan Gilbert / address@hidden / Manchester, UK
> 
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK