Re: [Qemu-block] [Qemu-devel] [PATCH 2/3] qemu-img: Use zero writes after source backing EOF

[Eric Blake]

On 7/13/18 6:14 AM, Max Reitz wrote:
> Past the end of the source backing file, we memset() buf_old to zero, so
> it is clearly easy to use blk_pwrite_zeroes() instead of blk_pwrite()
> then.
> 
> Signed-off-by: Max Reitz <address@hidden>
> ---
>  qemu-img.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 

> @@ -3458,8 +3461,12 @@ static int img_rebase(int argc, char **argv)
>                  if (compare_buffers(buf_old + written, buf_new + written,
>                                      n - written, &pnum))
>                  {
> -                    ret = blk_pwrite(blk, offset + written,
> -                                     buf_old + written, pnum, 0);
> +                    if (buf_old_is_zero) {
> +                        ret = blk_pwrite_zeroes(blk, offset + written, pnum, 
> 0);

Should we allow BDRV_REQ_MAY_UNMAP here, either unconditionally, or
based on a command line knob that told us whether the user is more
interested in a sparse result (that still reads as zero) vs. a
fully-allocated result?

Reviewed-by: Eric Blake <address@hidden>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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