Re: [Qemu-devel] [PATCH v9 07/11] block: Add QMP support for streaming to an intermediate layer

[Kevin Wolf]

Am 28.04.2016 um 14:20 hat Alberto Garcia geschrieben:
> On Wed 27 Apr 2016 03:34:19 PM CEST, Max Reitz <address@hidden> wrote:
> >> +    /* Look for the top-level node that contains 'bs' in its chain */
> >> +    active = NULL;
> >> +    do {
> >> +        active = bdrv_next(active);
> >> +    } while (active && !bdrv_chain_contains(active, bs));
> >
> > Alternatively, you could iterate up directly from @bs. Just look for
> > the BdrvChild in bs->parents with .role == &child_backing.
> 
> Yes, but the BdrvChild in bs->parents does not contain any pointer to
> the actual parent node, so unless we want to change that structure I
> wouldn't go for this solution.

Yes, and that's intentional. If we do things right, there shouldn't be
any need to go upwards in the tree. Our current op blockers are not done
right as they are only effective when set on the top layer. Some
temporary ugliness to deal with this is okay; breaking the assumption
that children don't know their parents would require much better
justification.

> > More interesting question: What happens if you have e.g. a qcow2 file
> > as a quorum child, and then want to stream inside of the qcow2 backing
> > chain?
> >
> > So maybe you should first walk up along &child_backing and then along
> > &child_file/&child_format. I think a generic bdrv_get_root_bs()
> > wouldn't hurt.
> 
> You're right. I'm not sure if that would have other consequences that we
> should consider, so maybe we can add that later, with its own set of
> tests.
> 
> Also, this is all necessary because of the problem with bdrv_reopen().
> If that is ever fixed there's no need to block the active layer at all.

This patch errors out if we can't find the active layer. Sounds safe and
appropriate for an initial version. The real solution isn't to improve
the magic to find the root node, but to remove the need to find it (by
getting the new op blockers).

Kevin