On Tue, Jun 22, 2021 at 6:38 PM Kevin Wolf <kw...@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 <ebl...@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.
