01.10.2019 12:54, 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. > > 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.
The problem is that mirror filter is already in Rhel qemu and we have this bug, but libvirt is not yet prepared to match node-name on migration, or am I wrong? > >>> A simple way to do this might be a "child_unfiltered" BdrvChild role >>> that simply bypasses the filter that was inserted and serves no real >>> purpose other than to allow the child to have a parent link and find who >>> it's """real""" parent is. >>> >>> Because of flushing, reopen, sync, drain &c &c &c I'm not sure how >>> feasible this quick idea might be, though. >>> >>> >>> - Corollary fix #1: call error_setg if the bitmap node name that's about >>> to go over the wire is an autogenerated node: this is never correct! >>> >>> (Why not? because the target is incapable of matching the node-name >>> because they are randomly generated AND you cannot specify node-names >>> with # prefixes as they are especially reserved! >>> >>> (This raises a related problem: if you explicitly add bitmaps to nodes >>> with autogenerated names, you will be unable to migrate them.)) >>> >> >> In other words, we need a well defined way to match nodes on source and >> destination, >> keeping in mind filters, to migrate bitmaps correctly. >> >> Hm, did you thought about bitmaps in filters? It's not a problem to create >> bitmap in >> mirror-top filter during mirror job:) >> >> Or what about bitmaps in Quorum children? Or what about bitmap in qcow2 file >> child bs? >> >> If node-names are different on source and destination, what is the same? Top >> blk name >> and bdrv-children names (I recently saw Max's idea to check node "path" in >> iotest). > > blk_name has to be assumed to be "". The BdrvChild path changes when > filters are inserted (and inserting filters on the destination that > aren't present on the source, or vice versa, sounds like something that > should just work). Skipping filters is not a problem until we don't want bitmaps in filters. > > So both parts of this are not great ways for addressing nodes. Not great, yes.. > >> So, actually node is migration-addressable, if path >> <blk-name>/root[/child-name] to the >> defines this node directly (we must not have children with same name for >> some node in >> the path). >> >> And I think it's a correct way to define node in migration stream - by path. > > I'm afraid node-name is the only thing that could possibly work reliably > for identifying nodes. > Can we do it to fix bug in current Rhel's qemu? How much effort on libvirt part is needed? -- Best regards, Vladimir
