Re: [Qemu-devel] [PATCHv3 1.8 1/9] qemu-img: add support for skipping zeroes in input during convert

[Stefan Hajnoczi]

On Wed, Nov 27, 2013 at 11:07:01AM +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) {
> +                    if (!(ret & BDRV_BLOCK_DATA)) {
> +                        sector_num += n1;
> +                        continue;
> +                    }
> +                    /* The next 'n1' sectors are allocated in the input 
> image.
> +                     * Copy only those as they may be followed by unallocated
> +                     * sectors. */
> +                    nb_sectors = n1;
> +                }
> +                /* avoid redundant callouts to get_block_status */
> +                sector_num_next_status = sector_num + n1;

Can you explain when we need sector_num_next_status?  It's not clear to
me from this patch when we will loop around already knowing that blocks
are allocated.