Re: [Qemu-devel] [PATCH v3 13/18] block: introduce new filter driver: fleecing-hook

[Kevin Wolf <kwolf@redhat.com>]

Hi Vladimir,

can you please check your mailer settings? The plain text version of the
emails is hardly legible because it mixes quotes text and replies. I
had to manually open the HTML part to figure out what you really wrote.

Am 05.10.2018 um 17:00 hat Vladimir Sementsov-Ogievskiy geschrieben:
> Hmm, how to share children?
> 
> backup job has two source BdrvChild'ren - child_job and child_root of
> job blk and two target BdrvChild'ren - again, child_job and
> child_root.
> 
> backup_top has source child - child_backing and second - child_file
> (named "target")..

Right, these are six BdrvChild instances in total. I think we can ignore
the child_job ones, they are internal to the block job infrastructure,
so we have four of them left.

> Which BdrvChild'ren you suggest to remove? They are all different.

Now that you introduced backup_top, I think we don't need any
BlockBackends any more. So I suggest to remove the child_root ones and
to do all I/O through the child_backing and child_file ones of
backup_top.

> I don't know, why job needs both unnamed blk's and child_job's, and I
> don't know is it necessary for backup to use blk's not BdrvChild'ren..

I think we had a case recently where it turned out that it is strictly
speaking even wrong for jobs to use BlockBackends in a function that
intercepts a request on the BDS level (like the copy-before-write of
backup).

So getting rid of the BlockBackends isn't only okay, but actually a good
thing by itself.

> And with internal way in none-mode we'll have two unused blk's  and
> four unused BdrvChild'ren.. Or we want to rewrite backup to use
> BdrvChild'ren for io operations and drop child_job BdrvChild'ren? So
> I'm lost. What did you mean?

child_job isn't actually unused, even though you never use them to make
requests. The child_job BdrvChild is important because of the
BdrvChildRole callbacks it provides to the block job infrastructure.

Kevin