Am 10.04.2018 um 12:40 hat Dr. David Alan Gilbert geschrieben: > * 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.
I don't think so. In theory, both options can provide the same. If anything, it's related specifically to the phases that Jirka described that libvirt uses to implement migration. > > 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? The key is making the delayed activation of block devices (and probably delayed announcement of NICs? - you didn't answer that part) optional instead of making it the default. We can use Jirka's suggestion of adding a migration capability that enables it, or I suppose a new option to -incoming could work, too. It doesn't really matter what the syntax is, but the management tool must request it explicitly. Kevin
