Re: [Qemu-block] [Qemu-devel] [PATCH for-4.1] qcow2: Allow -o compat=v3 during qemu-img amend

[Eric Blake]

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 <address@hidden>
>> ---
>>
>> 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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