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One clear problem with how qcow2's refcount structure rebuild algorithm
used to be before "qcow2: Improve refcount structure rebuilding" was
that it is prone to failure for qcow2 images on block devices: There is
generally unused space after the actual image, and if that exceeds what
one refblock covers, the old algorithm would invariably write the
reftable past the block device's end, which cannot work. The new
algorithm does not have this problem.
Test it with three tests:
(1) Create an image with more empty space at the end than what one
refblock covers, see whether rebuilding the refcount structures
results in a change in the image file length. (It should not.)
(2) Leave precisely enough space somewhere at the beginning of the image
for the new reftable (and the refblock for that place), see whether
the new algorithm puts the reftable there. (It should.)
(3) Test the original problem: Create (something like) a block device
with a fixed size, then create a qcow2 image in there, write some
data, and then have qemu-img check rebuild the refcount structures.
Before HEAD^, the reftable would have been written past the image
file end, i.e. outside of what the block device provides, which
cannot work. HEAD^ should have fixed that.
("Something like a block device" means a loop device if we can use
one ("sudo -n losetup" works), or a FUSE block export with
growable=false otherwise.)
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20220405134652.19278-3-hreitz@redhat.com>
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