On 7/5/19 10:53 AM, Eric Blake wrote: > On 7/5/19 10:28 AM, Eric Blake wrote: >> Commit b76b4f60 allowed '-o compat=v3' as an alias for the >> less-appealing '-o compat=1.1' for 'qemu-img create' since we want to >> use the QMP form as much as possible, but forgot to do likewise for >> qemu-img amend. Also, it doesn't help that '-o help' doesn't list our >> new preferred spellings. >> >> Signed-off-by: Eric Blake <ebl...@redhat.com> >> --- >> >> I'm arguing that the lack of consistency is a bug, even though the bug >> has been present since 2.12. > > I found this bug while chasing down another one: trying to see if we can > now lift our restriction against 'qemu-img resize' on an image with > internal snapshots. For v3 images, the limitation is artificial (the > spec says every snapshot is required to have an associated size, so you > know what size to change back to when reverting to that snapshot); but > for v2 the limitation is real (the spec did not require tracking image > size, and therefore changing the size meant that you might not be able > to safely revert). Except that we ALSO have a bug in qemu-img amend: > > 1. Create a v2 file with internal snapshot. On CentOS 6: > $ qemu-img create -f qcow2 file 1m > $ qemu-img snapshot -c s1 file > 2. Check that the internal snapshot header uses the smaller size: > $ od -Ax -j64 -N8 -tx1 file # Learn the offset for the next command > $ offset=$((0x50000+36)) > $ od -Ax -j$offset -N 4 -tx1 file > => extra field is 0 > 3. Upgrade it to v3. Using qemu.git master: > $ qemu-img amend -o compat=1.1 file > 4. Check the internal snapshot header size: > $ od -Ax -j64 -N8 -tx1 file # Learn the offset for the next command > $ offset=$((0x50000+36)) > $ od -Ax -j$offset -N 4 -tx1 file > => oops - extra field is still 0, but should now be at least 16. >
Oh, and 'qemu-img check file' fails to diagnose the v3 image as violating the qcow2 spec. -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org
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