Re: bitmap migration bug with -drive while block mirror runs

[Vladimir Sementsov-Ogievskiy]

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