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From: "Denis V. Lunev" <den-lists@virtuozzo.com>
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Date: Thu, 21 Apr 2016 09:53:18 +0300
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Subject: Re: [Qemu-devel] [PATCH v3] qemu-img: check block status of backing
 file when converting.
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To: Ren Kimura <rkx1209dev@gmail.com>, mreitz@redhat.com
Cc: kwolf@redhat.com, qemu-devel@nongnu.org, qemu-block@nongnu.org

On 04/17/2016 07:30 AM, Ren Kimura wrote:
> When converting images, check the block status of its backing file chain
> to avoid needlessly reading zeros.
>
> Signed-off-by: Ren Kimura <rkx1209dev@gmail.com>

IMHO bdrv_get_block_status_above(bs, NULL, ...) should do the job 
without recursion.