Am 01.10.2019 um 18:12 hat Vladimir Sementsov-Ogievskiy geschrieben: > 01.10.2019 18:58, Kevin Wolf wrote: > > Am 01.10.2019 um 17:09 hat John Snow geschrieben: > >> > >> > >> On 10/1/19 5:54 AM, Kevin Wolf wrote: > >>> Am 01.10.2019 um 10:57 hat Vladimir Sementsov-Ogievskiy geschrieben: > >>>> 01.10.2019 3:09, John Snow wrote: > >>>>> Hi folks, I identified a problem with the migration code that Red Hat QE > >>>>> found and thought you'd like to see it: > >>>>> > >>>>> https://bugzilla.redhat.com/show_bug.cgi?id=1652424#c20 > >>>>> > >>>>> Very, very briefly: drive-mirror inserts a filter node that changes what > >>>>> bdrv_get_device_or_node_name() returns, which causes a migration > >>>>> problem. > >>>>> > >>>>> > >>>>> Ignorant question #1: Can we multi-parent the filter node and > >>>>> source-node? It looks like at the moment both consider their only parent > >>>>> to be the block-job and don't have a link back to their parents > >>>>> otherwise. > >>>>> > >>>>> > >>>>> Otherwise: I have a lot of cloudy ideas on how to solve this, but > >>>>> ultimately what we want is to be able to find the "addressable" name for > >>>>> the node the bitmap is attached to, which would be the name of the first > >>>>> ancestor node that isn't a filter. (OR, the name of the block-backend > >>>>> above that node.) > >>>> > >>>> > >>>> Better would be to migrate by node-name only.. But am I right that > >>>> node-names are different on source and destination? Or this situation > >>>> changed? > >>> > >>> Traditionally, I think migration assumes that frontends (guest devices) > >>> must match exactly, but backends may and usually will differ. > >>> > >>> Of course, dirty bitmaps are a backend feature that isn't really related > >>> to guest devices, so this doesn't really work out any more in your case. > >>> BlockBackend names are unusable for this purpose (especially as we're > >>> moving towards anonymous BlockBackends everywhere), which I guess > >>> essentially means node-name is the only option left. > >>> > >> > >> The problem as I see it involves API stability. > >> > >> We allow block-dirty-bitmap-add against e.g. "drive1" through the > >> block-backend name (the name of the "drive" as the user sees it.) > >> > >> Of course, once you start mirror, you aren't able to access that bitmap > >> through that namepair anymore -- the "address" of the bitmap has "changed"! > >> > >> (In actual fact, the bitmap always had two addresses; and simply we lost > >> an alias -- but it's the one that the user likely used to create the > >> bitmap, so that's bad.) > > > > So if I understand correctly, the problem is that without a filter, in > > some setups we get a usable BlockBackend name like "drive1", but when a > > filter is added, we return the node-name instead which is > > auto-generated and will be different on the destination. > > > > Looking at the ChildRole documentation: > > > > /* Returns a name that is supposedly more useful for human users than > > the > > * node name for identifying the node in question (in particular, a BB > > * name), or NULL if the parent can't provide a better name. */ > > const char *(*get_name)(BdrvChild *child); > > > > I'd argue that a BlockBackend name is more useful for a human user even > > across filter, so I'd support a .get_name implementation for a filter > > child role (which Max wanted to introduce anyway for his filter series). > > > > Of course, if you have a function that is made to return a convenient > > text for human users, and you use it for a stable ABI like the migration > > stream, this is an idea that would certainly have caused an entertaining > > Linus rant in the good old kernel times. > > > >>> Is bitmap migration something that must be enabled explicitly or does > >>> it happen automatically? If it's explicit, then making an additional > >>> requirement (matching node-names) shouldn't be a problem. > >> > >> This means that bitmap migration becomes a blockdev-only feature. > > > > I meant this more as the preferred way for the future rather than the > > only thing supported. > > > > But Peter has actually mentioned that for libvirt it will be > > blockdev-only anyway. So do we even have a good reason to invest much > > for the non-blockdev case? > > > > Maybe making it blockdev-only is actually pretty reasonable. > > We in Virtuozzo use bitmap migration, so I'd have to fix it at least > downstream (it seems easier than switch downstream libvirt to blockdev now). > > And what about original bug > https://bugzilla.redhat.com/show_bug.cgi?id=1652424#c20 > ? > > Also, if we make it blockdev-only upstream, what we mean by that? Node names > on destination must match, or we add additional qmp command > migration-set-bitmap-node-mapping, to specify mapping between node names on > source and target?
Good question. :-) I would have thought that just having matching node-names would be pretty convenient for users, but Peter seems to disagree. With a separate migration-set-bitmap-node-mapping, what would we do if migrate is called before a mapping is configured? Would this cause migration failure? I would find that pretty heavy. Maybe default to a 1:1 mapping and allow the user to override it? (And if so, do we do the mapping on the source or the destination?) Kevin
