Re: [PATCH] qemu-img convert: Don't pre-zero images

[Nir Soffer <nsoffer@redhat.com>]

On Mon, Jun 22, 2020 at 6:12 PM Kevin Wolf <kwolf@redhat.com> wrote:
>
> Since commit 5a37b60a61c, qemu-img create will pre-zero the target image
> if it isn't already zero-initialised (most importantly, for host block
> devices, but also iscsi etc.), so that writing explicit zeros wouldn't
> be necessary later.
>
> This could speed up the operation significantly, in particular when the
> source image file was only sparsely populated. However, it also means
> that some block are written twice: Once when pre-zeroing them, and then
> when they are overwritten with actual data. On a full image, the
> pre-zeroing is wasted work because everything will be overwritten.
>
> In practice, write_zeroes typically turns out faster than writing
> explicit zero buffers, but slow enough that first zeroing everything and
> then overwriting parts can be a significant net loss.
>
> Meanwhile, qemu-img convert was rewritten in 690c7301600 and zero blocks
> are now written to the target using bdrv_co_pwrite_zeroes() if the
> target could be pre-zeroed. This way we already make use of the faster
> write_zeroes operation, but avoid writing any blocks twice.
>
> Remove the pre-zeroing because these days this former optimisation has
> actually turned into a pessimisation in the common case.
>
> Reported-by: Nir Soffer <nsoffer@redhat.com>
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>

Thanks, you can also add:

    Tested-by:  Nir Soffer <nsoffer@redhat.com>

> ---
>  qemu-img.c | 9 ---------
>  1 file changed, 9 deletions(-)
>
> diff --git a/qemu-img.c b/qemu-img.c
> index d7e846e607..bdb9f6aa46 100644
> --- a/qemu-img.c
> +++ b/qemu-img.c
> @@ -2084,15 +2084,6 @@ static int convert_do_copy(ImgConvertState *s)
>          s->has_zero_init = bdrv_has_zero_init(blk_bs(s->target));
>      }
>
> -    if (!s->has_zero_init && !s->target_has_backing &&
> -        bdrv_can_write_zeroes_with_unmap(blk_bs(s->target)))
> -    {
> -        ret = blk_make_zero(s->target, BDRV_REQ_MAY_UNMAP | BDRV_REQ_NO_FALLBACK);
> -        if (ret == 0) {
> -            s->has_zero_init = true;
> -        }
> -    }
> -
>      /* Allocate buffer for copied data. For compressed images, only one cluster
>       * can be copied at a time. */
>      if (s->compressed) {
> --
> 2.25.4
>