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Re: [Qemu-devel] [PATCH v4 1/1] qemu-img: check block status of backing
From: |
Max Reitz |
Subject: |
Re: [Qemu-devel] [PATCH v4 1/1] qemu-img: check block status of backing file when converting. |
Date: |
Wed, 27 Apr 2016 13:45:26 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.7.2 |
On 23.04.2016 08:26, 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 <address@hidden>
> ---
> qemu-img.c | 31 +++++++++++++++++++++++++++++--
> 1 file changed, 29 insertions(+), 2 deletions(-)
>
> diff --git a/qemu-img.c b/qemu-img.c
> index 06264d9..b771227 100644
> --- a/qemu-img.c
> +++ b/qemu-img.c
> @@ -1451,6 +1451,22 @@ static void convert_select_part(ImgConvertState *s,
> int64_t sector_num)
> }
> }
>
> +static int64_t get_backing_status(BlockDriverState *bs,
> + int64_t sector_num,
> + int nb_sectors, int *pnum)
> +{
> + while (bs->backing) {
> + int64_t ret;
> + BlockDriverState *file;
> + bs = bs->backing->bs;
> + ret = bdrv_get_block_status(bs, sector_num, nb_sectors, pnum, &file);
> + if (ret < 0 || ret & (BDRV_BLOCK_DATA | BDRV_BLOCK_ZERO)) {
> + return ret;
> + }
Denis' suggestion was to basically drop this function and call
bdrv_get_block_status_above() instead.
I wouldn't strictly oppose keeping this function because of the
BDRV_BLOCK_{DATA,ZERO} vs. BDRV_BLOCK_ALLOCATED issue I talked about before.
One thing we can definitely learn from bdrv_get_block_status_above(),
though, is that a
nb_sectors = MIN(nb_sectors, *pnum);
is missing here (see bdrv_co_get_block_status_above()).
(Sorry for not noticing this before)
> + }
> + return -1;
> +}
> +
> static int convert_iteration_sectors(ImgConvertState *s, int64_t sector_num)
> {
> int64_t ret;
> @@ -1477,10 +1493,21 @@ static int convert_iteration_sectors(ImgConvertState
> *s, int64_t sector_num)
> } else if (!s->target_has_backing) {
> /* Without a target backing file we must copy over the contents
> of
> * the backing file as well. */
> - /* TODO Check block status of the backing file chain to avoid
> + /* Check block status of the backing file chain to avoid
> * needlessly reading zeroes and limiting the iteration to the
> * buffer size */
> - s->status = BLK_DATA;
> + ret = get_backing_status(blk_bs(s->src[s->src_cur]),
> + sector_num - s->src_cur_offset,
> + n, &n);
In any case, I think I agree with Denis that just calling
ret = bdrv_get_block_status_above(blk_bs(s->src[s->src_cur]), NULL,
sector_num - s->src_cur_offset, n, &n,
&file);
would be sufficient.
(Sorry for not noticing this either...)
Max
> + if (ret < 0) {
> + return ret;
> + }
> +
> + if (ret & BDRV_BLOCK_ZERO) {
> + s->status = BLK_ZERO;
> + } else {
> + s->status = BLK_DATA;
> + }
> } else {
> s->status = BLK_BACKING_FILE;
> }
>
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