* Kevin Wolf (kw...@redhat.com) wrote: > Am 10.04.2018 um 10:45 hat Dr. David Alan Gilbert geschrieben: > > * Kevin Wolf (kw...@redhat.com) wrote: > > > Am 10.04.2018 um 09:36 hat Jiri Denemark geschrieben: > > > > On Mon, Apr 09, 2018 at 15:40:03 +0200, Kevin Wolf wrote: > > > > > Am 09.04.2018 um 12:27 hat Dr. David Alan Gilbert geschrieben: > > > > > > It's a fairly hairy failure case they had; if I remember correctly > > > > > > it's: > > > > > > a) Start migration > > > > > > b) Migration gets to completion point > > > > > > c) Destination is still paused > > > > > > d) Libvirt is restarted on the source > > > > > > e) Since libvirt was restarted it fails the migration (and hence > > > > > > knows > > > > > > the destination won't be started) > > > > > > f) It now tries to resume the qemu on the source > > > > > > > > > > > > (f) fails because (b) caused the locks to be taken on the > > > > > > destination; > > > > > > hence this patch stops doing that. It's a case we don't really > > > > > > think > > > > > > about - i.e. that the migration has actually completed and all the > > > > > > data > > > > > > is on the destination, but libvirt decides for some other reason to > > > > > > abandon migration. > > > > > > > > > > If you do remember correctly, that scenario doesn't feel tricky at > > > > > all. > > > > > libvirt needs to quit the destination qemu, which will inactivate the > > > > > images on the destination and release the lock, and then it can > > > > > continue > > > > > the source. > > > > > > > > > > In fact, this is so straightforward that I wonder what else libvirt is > > > > > doing. Is the destination qemu only shut down after trying to continue > > > > > the source? That would be libvirt using the wrong order of steps. > > > > > > > > There's no connection between the two libvirt daemons in the case we're > > > > talking about so they can't really synchronize the actions. The > > > > destination daemon will kill the new QEMU process and the source will > > > > resume the old one, but the order is completely random. > > > > > > Hm, okay... > > > > > > > > > Yes it was a 'block-activate' that I'd wondered about. One > > > > > > complication > > > > > > is that if this now under the control of the management layer then > > > > > > we > > > > > > should stop asserting when the block devices aren't in the expected > > > > > > state and just cleanly fail the command instead. > > > > > > > > > > Requiring an explicit 'block-activate' on the destination would be an > > > > > incompatible change, so you would have to introduce a new option for > > > > > that. 'block-inactivate' on the source feels a bit simpler. > > > > > > > > As I said in another email, the explicit block-activate command could > > > > depend on a migration capability similarly to how pre-switchover state > > > > works. > > > > > > Yeah, that's exactly the thing that we wouldn't need if we could use > > > 'block-inactivate' on the source instead. It feels a bit wrong to > > > design a more involved QEMU interface around the libvirt internals, > > > > It's not necessarily 'libvirt internals' - it's a case of them having to > > cope with recovering from failures that happen around migration; it's > > not an easy problem, and if they've got a way to stop both sides running > > at the same time that's pretty important. > > The 'libvirt internals' isn't that it needs an additional state where > neither source nor destination QEMU own the images, but that it has to > be between migration completion and image activation on the destination > rather than between image inactivation on the source and migration > completion. The latter would be much easier for qemu, but apparently it > doesn't work for libvirt because of how it works internally.
I suspect this is actually a fundamental requirement to ensuring that we don't end up with a QEMU running on both sides rather than how libvirt is structured. > But as I said, I'd just implement both for symmetry and then management > tools can pick whatever makes their life easier. > > > > but > > > as long as we implement both sides for symmetry and libvirt just happens > > > to pick the destination side for now, I think it's okay. > > > > > > By the way, are block devices the only thing that need to be explicitly > > > activated? For example, what about qemu_announce_self() for network > > > cards, do we need to delay that, too? > > > > > > In any case, I think this patch needs to be reverted for 2.12 because > > > it's wrong, and then we can create the proper solution in the 2.13 > > > timefrage. > > > > what case does this break? > > I'm a bit wary of reverting this, which fixes a known problem, on the > > basis that it causes a theoretical problem. > > It breaks the API. And the final design we're having in mind now is > compatible with the old API, not with the new one exposed by this patch, > so that switch would break the API again to get back to the old state. > > Do you know all the scripts that people are using around QEMU? I don't, > but I know that plenty of them exist, so I don't think we can declare > this API breakage purely theoretical. > > Yes, the patch fixes a known problem, but also a problem that is a rare > corner case error that you can only hit with really bad timing. Do we > really want to risk unconditionally breaking success cases for fixing a > mostly theoretical corner case error path (with the failure mode that > the guest is paused when it shouldn't be)? Hmm; having chatted to Jiri I'm OK with reverting it, on the condition that I actually understand how this alternative would work first. I can't currently see how a block-inactivate would be used. I also can't see how a block-activate unless it's also with the change that you're asking to revert. Can you explain the way you see it working? Dave > Kevin -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
