Re: [Qemu-devel] [RFC][PATCH] qemu-img: add support for skipping zeroes in input during convert

[Stefan Hajnoczi]

On Fri, Nov 22, 2013 at 03:48:30PM +0100, Peter Lieven wrote:
> -            /* If the output image is being created as a copy on write image,
> -               assume that sectors which are unallocated in the input image
> -               are present in both the output's and input's base images (no
> -               need to copy them). */
> -            if (out_baseimg) {
> -                ret = bdrv_is_allocated(bs[bs_i], sector_num - bs_offset,
> -                                        n, &n1);

int coroutine_fn bdrv_is_allocated(BlockDriverState *bs, int64_t sector_num,
                                   int nb_sectors, int *pnum)
{
    int64_t ret = bdrv_get_block_status(bs, sector_num, nb_sectors, pnum);
    if (ret < 0) {
        return ret;
    }
    return
        (ret & BDRV_BLOCK_DATA) ||
        ((ret & BDRV_BLOCK_ZERO) && !bdrv_has_zero_init(bs));
}

bdrv_has_zero_init() returns false when bs->backing_hd != NULL.

> +            if (out_baseimg || has_zero_init) {
> +                n = nb_sectors > INT_MAX ? INT_MAX : nb_sectors;
> +                ret = bdrv_get_block_status(bs[bs_i], sector_num - bs_offset,
> +                                            n, &n1);
>                  if (ret < 0) {
> -                    error_report("error while reading metadata for sector "
> -                                 "%" PRId64 ": %s",
> +                    error_report("error while reading block status of sector 
> %" PRId64 ": %s",
>                                   sector_num - bs_offset, strerror(-ret));
>                      goto out;
>                  }
> -                if (!ret) {
> +                /* If the output image is zero initialized, we are not 
> working
> +                 * on a shared base and the input is zero we can skip the 
> next
> +                 * n1 bytes */
> +                if (!out_baseimg && has_zero_init && ret & BDRV_BLOCK_ZERO) {
> +                    sector_num += n1;
> +                    continue;
> +                }
> +                /* If the output image is being created as a copy on write 
> image,
> +                   assume that sectors which are unallocated in the input 
> image
> +                   are present in both the output's and input's base images 
> (no
> +                   need to copy them). */
> +                if (out_baseimg && !(ret & BDRV_BLOCK_DATA)) {
>                      sector_num += n1;
>                      continue;
>                  }

How are these two if statements different from bdrv_is_allocated()?

Stefan