Re: [PATCH v8 33/34] qcow2: Assert that expand_zero_clusters_in_l1() does not support subclusters

[Eric Blake]

On 6/10/20 10:03 AM, Alberto Garcia wrote:
> This function is only used by qcow2_expand_zero_clusters() to
> downgrade a qcow2 image to a previous version. It is however not
> possible to downgrade an image with extended L2 entries because older
> versions of qcow2 do not have this feature.

Well, it _is_ possible, but it would involve rewriting the entire L1/L2 
tables (including all internal snapshots), as well as causing I/O to COW 
every cluster where not all subclusters are allocated; and doing that 
conversion while remaining crash-consistent requires some thought and a 
temporary extra load on disk space (we can't discard the old table until 
the new one is completely written).

It would be more accurate to merely state that we are not prepared to 
implement it at this time.

> Signed-off-by: Alberto Garcia <berto@igalia.com>
> Reviewed-by: Eric Blake <eblake@redhat.com>
> ---
>   block/qcow2-cluster.c      | 8 +++++++-
>   tests/qemu-iotests/061     | 6 ++++++
>   tests/qemu-iotests/061.out | 5 +++++
>   3 files changed, 18 insertions(+), 1 deletion(-)

Whether or not we update the commit message, R-b stands for the code.

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org