[0/4] qcow2: Improve refcount structure rebuilding

Message

Max Reitz March 10, 2021, 3:59 p.m. UTC

Hi,

When the qcow2 refcount structure is broken to a point where we cannot
rely on any information from it (because it shows clusters as free that
are in use), “qemu-img check -r all” completely rewrites it.

The new reftable is preferably written into the area covered by the last
refblock for the image, but if that refblock is empty (e.g. because the
image is on a block device and there is just nothing near the end of the
block device), then the reftable will be put after the image’s end.
Which is a problem on block devices, because they can’t easily be
resized (also, resizing wouldn’t really help in this case, because the
reftable would still be written past the new end).

So patch 1 modifies the algorithm to put the reftable into the first
free space in the image, and patch 4 adds a test.  So that we can make
patch 4 a bit nicer, patches 2 and 3 are included.  (In case you don’t
like anything about 2 or 3, I also have a version of this series without
patches 2 and 3, where 4 is correspondingly unnicer.)


Max Reitz (4):
  qcow2: Improve refcount structure rebuilding
  iotests/common.qemu: Add _cleanup_single_qemu
  iotests/common.qemu: Allow using the QSD
  iotests/108: Test new refcount rebuild algorithm

 block/qcow2-refcount.c         | 126 ++++++++--------
 tests/qemu-iotests/108         | 259 +++++++++++++++++++++++++++++++++
 tests/qemu-iotests/108.out     |  80 ++++++++++
 tests/qemu-iotests/common.qemu | 108 ++++++++++----
 4 files changed, 483 insertions(+), 90 deletions(-)