Re: [Qemu-devel] [PATCH for-4.0?] qemu-img: Enable BDRV_REQ_MAY_UNMAP in convert

[Eric Blake]

On 3/23/19 5:59 PM, Nir Soffer wrote:
> With Kevin's "block: Fix slow pre-zeroing in qemu-img convert"[1]
> we skip the pre zero step called like this:
> 
>     blk_make_zero(s->target, BDRV_REQ_MAY_UNMAP | BDRV_REQ_NO_FALLBACK)
> 
> And we write zeroes later using:
> 
>     blk_co_pwrite_zeroes(s->target,
>                          sector_num << BDRV_SECTOR_BITS,
>                          n << BDRV_SECTOR_BITS, 0);
> 
> Since we use flags=0, this is translated to NBD_CMD_WRITE_ZEROES with
> NBD_CMD_FLAG_NO_HOLE flag, which cause the NBD server to allocated space
> instead of punching a hole.
> 

> 
> $ qemu-img info dst.img
> virtual size: 50M (52428800 bytes)
> disk size: 5.0M

Up to here makes sense for the commit message...

> 
> Tested on top of Kevin patches:
> http://lists.nongnu.org/archive/html/qemu-block/2019-03/msg00755.html
> 
> I'm not sure this change is correct for all cases, posting for
> discussion.

...this part probably belongs...

> 
> [1] http://lists.nongnu.org/archive/html/qemu-block/2019-03/msg00761.html
> 
> Signed-off-by: Nir Soffer <address@hidden>
> ---


...here, after the ---, as it is useful for reviewers but not needed for
the permanent git log.

>  qemu-img.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/qemu-img.c b/qemu-img.c
> index 8ee63daeae..ca9deb3758 100644
> --- a/qemu-img.c
> +++ b/qemu-img.c
> @@ -1752,11 +1752,12 @@ static int coroutine_fn 
> convert_co_write(ImgConvertState *s, int64_t sector_num,
>                  assert(!s->target_has_backing);
>                  break;
>              }
>              ret = blk_co_pwrite_zeroes(s->target,
>                                         sector_num << BDRV_SECTOR_BITS,
> -                                       n << BDRV_SECTOR_BITS, 0);
> +                                       n << BDRV_SECTOR_BITS,
> +                                       BDRV_REQ_MAY_UNMAP);

The idea makes sense to me. Maybe Kevin will come up with a
counterargument why we don't want convert to allow holes by default, but
unless we have a strong argument against it, it looks like a performance
improvement worth having in 4.0.

Reviewed-by: Eric Blake <address@hidden>

>              if (ret < 0) {
>                  return ret;
>              }
>              break;
>          }
> 

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

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