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, 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. Kevin