Re: [Qemu-devel] [PATCH for-1.7 1/2] block/drive-mirror: Reuse backing HD for sync=none

[Eric Blake <eblake@redhat.com>]

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On 11/25/2013 12:28 PM, Max Reitz wrote:
> For "none" sync mode in "absolute-paths" mode, the current image should
> be used as the backing file for the newly created image.
> 
> The current behavior is:
> a) If the image to be mirrored has a backing file, use that (which is
>    wrong, since the operations recorded by "none" are applied to the
>    image itself, not to its backing file).
> b) If the image to be mirrored lacks a backing file, the target doesn't
>    have one either (which is not really wrong, but not really right,
>    either; "none" records a set of operations executed on the image
>    file, therefore having no backing file to apply these operations on
>    seems rather pointless).
> 
> For a, this is clearly a bugfix. For b, it is still a bugfix, although
> it might break existing API - but since that case crashed qemu just
> three weeks ago (before 1452686495922b81d6cf43edf025c1aef15965c0), we
> can safely assume there is no such API relying on that case yet.
> 
> Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>  blockdev.c | 3 +++
>  1 file changed, 3 insertions(+)

Reviewed-by: Eric Blake <eblake@redhat.com>

And definitely agree that this is a bug fix worth having in 1.7, before
we bake in borked semantics that libvirt would have to work around
depending on qemu version.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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