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? > > 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). 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. -- Best regards, Vladimir
