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Re: [Qemu-devel] [PATCH for-1.7 1/2] block/drive-mirror: Reuse backing H
From: |
Eric Blake |
Subject: |
Re: [Qemu-devel] [PATCH for-1.7 1/2] block/drive-mirror: Reuse backing HD for sync=none |
Date: |
Mon, 25 Nov 2013 12:37:04 -0700 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.1.0 |
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 <address@hidden>
> Signed-off-by: Max Reitz <address@hidden>
> ---
> blockdev.c | 3 +++
> 1 file changed, 3 insertions(+)
Reviewed-by: Eric Blake <address@hidden>
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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