Re: [PATCH v2 0/5] fix migration with bitmaps and mirror

[Peter Krempa <pkrempa@redhat.com>]

On Thu, Dec 19, 2019 at 11:51:01 +0300, Vladimir Sementsov-Ogievskiy wrote:
> Hi all!
> 
> It's a continuation for
> "bitmap migration bug with -drive while block mirror runs"
> <315cff78-dcdb-a3ce-2742-da3cc9f0ca97@redhat.com>
> https://lists.gnu.org/archive/html/qemu-devel/2019-09/msg07241.html
> 
> The problem is that bitmaps migrated to node with same node-name or
> blk-parent name. And currently only the latter actually work in libvirt.
> And with mirror-top filter it doesn't work, because
> bdrv_get_device_or_node_name don't go through filters.

I want to point out that since libvirt-5.10 we use -blockdev to
configure the backend of storage devices with qemu-4.2 and later. This
means unfortunately that the BlockBackend of the drive does not have a
name any more and thus the above will not work even if you make the
lookup code to see through filters.

As I've pointed out separately node-names are not good idea to use for
matching either as they can be distinct on the destination of migration.

Having same node names for images during migration was not documented as
a requiremend and even if it was the case when the mirror job is used
the destination is a different image and thus having a different node
name is expected.

Since it's not documented, expect the same situation as with
autogenerated nodenames, the destination may have different node-names
and the same node-name may refer to a different image. Implicit matching
based on node-names is thus impossible.