Re: [Qemu-devel] [PATCH v4 1/1] qemu-img: check block status of backing file when converting.

[Max Reitz <mreitz@redhat.com>]

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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 <rkx1209dev@gmail.com>
> ---
>  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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