Re: [PATCH v2 2/1] qemu-img: Add "backing":true to unallocated map segments

[Nir Soffer]

On Tue, Jun 15, 2021 at 11:54 AM Vladimir Sementsov-Ogievskiy
<vsementsov@virtuozzo.com> wrote:
>
> 11.06.2021 22:03, Eric Blake wrote:
> > To save the user from having to check 'qemu-img info --backing-chain'
> > or other followup command to determine which "depth":n goes beyond the
> > chain, add a boolean field "backing" that is set only for unallocated
> > portions of the disk.
> >
> > Signed-off-by: Eric Blake<eblake@redhat.com>
> > ---
> >
> > Touches the same iotest output as 1/1.  If we decide that switching to
> > "depth":n+1 is too risky, and that the mere addition of "backing":true
> > while keeping "depth":n is good enough, then we'd have just one patch,
> > instead of this double churn.  Preferences?
>
> If change something, this one patch seems safer. Still, Nir said he don't use 
> qemu-img map, so probably we don't need to modify qemu-img at all? Even our 
> iotests change shows that this change may be incompatible with at least 
> tests..
>
> I'm not against the patch and don't have strict opinion.
>
> And what I really think, is that qemu-img is outdated thing and we'd better 
> develop QMP interface, which can be used with qemu binary or with 
> qemu-storage-daemon.

I don't think qemu-storage-daemon can replace qemu-img. Having an easy
to use command
line tool is important. Using qmp with qemu-storage-daemon sounds like
a better option
for programs that want ultimate control.

Adding only "backing: true" seems a safe change that should not break
existing users
and make qemu-img map better.

The tests are broken because they compare strings instead of parsing
the json. A program
parsing qemu-img json output will not be broken by adding a new key.

Nir