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

[Kevin Wolf <kwolf@redhat.com>]

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"?

Kevin