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Date: Mon, 25 May 2015 16:36:44 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v4 3/8] mirror: Do zero write on target if
 sectors not allocated
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To: Fam Zheng <famz@redhat.com>, qemu-devel@nongnu.org
Cc: Kevin Wolf <kwolf@redhat.com>, qemu-block@nongnu.org, Jeff Cody <jcody@redhat.com>, qemu-stable@nongnu.org, Stefan Hajnoczi <stefanha@redhat.com>, jsnow@redhat.com, wangxiaolong@ucloud.cn



On 22/05/2015 05:40, Fam Zheng wrote:
> +    ret = bdrv_get_block_status(source, NULL, sector_num, nb_sectors, &pnum);
> +    if (ret < 0 || pnum < nb_sectors ||
> +            (ret & BDRV_BLOCK_ALLOCATED && !(ret & BDRV_BLOCK_ZERO))) {
> +        bdrv_aio_readv(source, sector_num, &op->qiov, nb_sectors,
> +                       mirror_read_complete, op);
> +    } else if (ret & BDRV_BLOCK_ZERO) {
> +        bdrv_aio_write_zeroes(s->target, sector_num, op->nb_sectors,
> +                              s->unmap ? BDRV_REQ_MAY_UNMAP : 0,
> +                              mirror_write_complete, op);
> +    } else {
> +        assert(!(ret & BDRV_BLOCK_ALLOCATED));
> +        bdrv_aio_discard(s->target, sector_num, op->nb_sectors,
> +                         mirror_write_complete, op);
> +    }

This doesn't work if you have a backing file.  You want to test
BDRV_BLOCK_DATA, not BDRV_BLOCK_ALLOCATED.

On the other hand, if BDRV_BLOCK_ALLOCATED is nonzero, you need to
recurse on bs->backing_hd.  The logic is very similar to
bdrv_is_allocated_above, but you need to write bdrv_get_block_status_above.

Paolo