[Qemu-devel] [PATCH v2 0/3] qemu-img: Allow rebase with no input base

[Max Reitz]

This series allows using qemu-img rebase (without -u) on images that do
not have a backing file.  Right now, this fails with the rather cryptic
error message:

$ qemu-img rebase -b base.qcow2 foo.qcow2
qemu-img: Could not open old backing file '': The 'file' block driver requires 
a file name

Yeah, well, OK.

With how rebase currently works, this would lead to the overlay being
filled with zeroes, however.  This is where patch 2 comes in and instead
makes rebase use blk_pwrite_zeroes() whenever it handles an area past
the input’s backing file’s EOF.

(Note that additionally we could try to punch holes in the overlay
whenever it matches the new backing file, but that’s something I’ll put
off for later.  (We don’t even have a reliable method for punching holes
into an overlay yet, although I would like to have such because it could
make active commit more efficient.))


And patch 3 adds the usual test.


v2:
- Patch 1: Added example to the commit message, and made a note that
           this is about rebase without -u [Eric]


git backport-diff against v1:

Key:
[----] : patches are identical
[####] : number of functional differences between upstream/downstream patch
[down] : patch is downstream-only
The flags [FC] indicate (F)unctional and (C)ontextual differences, respectively

001/3:[----] [--] 'qemu-img: Allow rebase with no input base'
002/3:[----] [--] 'qemu-img: Use zero writes after source backing EOF'
003/3:[----] [--] 'iotests: Add test for rebase without input base'


Max Reitz (3):
  qemu-img: Allow rebase with no input base
  qemu-img: Use zero writes after source backing EOF
  iotests: Add test for rebase without input base

 qemu-img.c                 | 72 +++++++++++++++++++++++---------------
 tests/qemu-iotests/024     | 70 ++++++++++++++++++++++++++++++++++++
 tests/qemu-iotests/024.out | 37 ++++++++++++++++++++
 3 files changed, 150 insertions(+), 29 deletions(-)

-- 
2.20.1