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

[Nir Soffer <nsoffer@redhat.com>]

On Tue, Jun 22, 2021 at 6:38 PM Kevin Wolf <kwolf@redhat.com> wrote:
>
> Am 11.06.2021 um 21:03 hat Eric Blake geschrieben:
> > 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?
>
> I think the additional flag is better because it's guaranteed to be
> backwards compatible, and because you don't need to know the number of
> layers to infer whether a cluster was allocated in the whole backing
> chain. And by exposing ALLOCATED we definitely give access to the whole
> information that exists in QEMU.
>
> However, to continue with the bike shedding: I won't insist on
> "allocated" even if that is what the flag is called internally and
> consistency is usually helpful, but "backing" is misleading, too,
> because intuitively it doesn't cover the top layer or standalone images
> without a backing file. How about something like "present"?

Looks hard to document:

# @present: if present and false, the range is not allocated within the
#           backing chain (since 6.1)

And is not consistent with "offset". It would work better as:

# @present: if present, the range is allocated within the backing
#           chain (since 6.1)

Or:

# @absent: if present, the range is not allocated within the backing
#           chain (since 6.1)

This is used by libnbd now:
https://github.com/libguestfs/libnbd/commit/1d01d2ac4f6443b160b7d81119d555e1aaedb56d

But I'm fine with "backing", It is consistent with BLK_BACKING_FILE,
meaning this area exposes data from a backing file (if one exists).

We use "backing" internally to be consistent with future qemu-img.