On 2018-05-01 18:57, Max Reitz wrote: > This is the issue: > > $ qemu-img create -f qcow2 base.qcow2 512M > Formatting 'base.qcow2', fmt=qcow2 size=536870912 cluster_size=65536 > lazy_refcounts=off refcount_bits=16 > $ qemu-img create -f qcow2 -b base.qcow2 source.qcow2 1G > Formatting 'source.qcow2', fmt=qcow2 size=1073741824 backing_file=base.qcow2 > cluster_size=65536 lazy_refcounts=off refcount_bits=16 > $ qemu-img convert -O qcow2 -B base.qcow2 -o compat=0.10 \ > source.qcow2 target.qcow2 > $ qemu-img info target.qcow2 > image: target.qcow2 > file format: qcow2 > virtual size: 1.0G (1073741824 bytes) > disk size: 512M <-------------------- this here > cluster_size: 65536 > backing file: base.qcow2 > Format specific information: > compat: 0.10 > refcount bits: 16 > $ qemu-img map target.qcow2 > Offset Length Mapped to File > 0x20000000 0x20000000 0x50000 target.qcow2 > > So qemu-img convert sees that source.qcow2 contains only zeroes past the > end of base.qcow2 -- but that is not the backing file, so it will > explicitly write zeroes to target.qcow2. But that file is compat=0.10, > so it does not support efficient zero writes and will actually fill that > area with real zeroes. Hence the mapping, hence the disk size. > > However, we don't need to write zeroes to an image when it is > initialized to zeroes -- which the current qemu-img code says doesn't > happen for target images with backing files. But it does happen when > the target backing file is shorter than the target image, then the area > past the end of the backing file may indeed read as zeroes and we don't > need to write anything there. > > So the first patch in this series makes qemu-img convert detect that > case and handle zeroes past the end of the backing file as "fall back to > the backing file" (because that means zeroes, at least if unallocated > areas read as zeroes (which I presume they have to for formats > supporting backing files, but better be safe than sorry)), and the > second adds an iotest. > > > Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1527898 > > > Max Reitz (2): > qemu-img: Special post-backing convert handling > iotests: Test post-backing convert target behavior > > qemu-img.c | 26 +++++++++++++++++++++++++- > tests/qemu-iotests/122 | 42 ++++++++++++++++++++++++++++++++++++++++++ > tests/qemu-iotests/122.out | 18 ++++++++++++++++++ > 3 files changed, 85 insertions(+), 1 deletion(-)
Applied to my block branch. Max
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