Re: [Qemu-devel] [PATCH v7 3/8] mirror: Do zero write on target if sectors not allocated

[Kevin Wolf <kwolf@redhat.com>]

Am 08.06.2015 um 07:56 hat Fam Zheng geschrieben:
> If guest discards a source cluster, mirroring with bdrv_aio_readv is overkill.
> Some protocols do zero upon discard, where it's best to use
> bdrv_aio_write_zeroes, otherwise, bdrv_aio_discard will be enough.
> 
> Signed-off-by: Fam Zheng <famz@redhat.com>
> ---
>  block/mirror.c | 20 ++++++++++++++++++--
>  1 file changed, 18 insertions(+), 2 deletions(-)
> 
> diff --git a/block/mirror.c b/block/mirror.c
> index d2515c7..3c38695 100644
> --- a/block/mirror.c
> +++ b/block/mirror.c
> @@ -164,6 +164,8 @@ static uint64_t coroutine_fn mirror_iteration(MirrorBlockJob *s)
>      int64_t end, sector_num, next_chunk, next_sector, hbitmap_next_sector;
>      uint64_t delay_ns = 0;
>      MirrorOp *op;
> +    int pnum;
> +    int64_t ret;
>  
>      s->sector_num = hbitmap_iter_next(&s->hbi);
>      if (s->sector_num < 0) {
> @@ -290,8 +292,22 @@ static uint64_t coroutine_fn mirror_iteration(MirrorBlockJob *s)
>      s->in_flight++;
>      s->sectors_in_flight += nb_sectors;
>      trace_mirror_one_iteration(s, sector_num, nb_sectors);
> -    bdrv_aio_readv(source, sector_num, &op->qiov, nb_sectors,
> -                   mirror_read_complete, op);
> +
> +    ret = bdrv_get_block_status_above(source, NULL, sector_num,
> +                                      nb_sectors, &pnum);
> +    if (ret < 0 || pnum < nb_sectors ||

Earlier today I told Richard Jones that qemu-img commit should really
be using zero cluster support in the backing file since 2.4 because I
remembered this commit. Turns out it doesn't actually use it but writes
explicit zeros instead.

The reason is the condition 'pnum < nb_sectors' here, which makes mirror
fall back to explicit writes if bdrv_get_block_status_above() doesn't
return enough sectors (enough being relatively large here, I think in
qemu-img commit it's always the full 10 MB buffer).

In other words, we are ignoring any zero areas smaller than 10 MB!

(What made this worse is that qcow2 had a bug that reports only a single
zero cluster at a time, so it would never report more than 10 MB, even
if the image was completely zeroed. I've sent a fix for that one.)

In order to fix this, we'll probably need to move the call to
bdrv_get_block_status_above() before actually allocating memory and
all that for the full nb_chunks. We should detect zeros on the usual
block job granularity (64k by default, I think).

> +            (ret & BDRV_BLOCK_DATA && !(ret & BDRV_BLOCK_ZERO))) {
> +        bdrv_aio_readv(source, sector_num, &op->qiov, nb_sectors,
> +                       mirror_read_complete, op);
> +    } else if (ret & BDRV_BLOCK_ZERO) {
> +        bdrv_aio_write_zeroes(s->target, sector_num, op->nb_sectors,
> +                              s->unmap ? BDRV_REQ_MAY_UNMAP : 0,
> +                              mirror_write_complete, op);
> +    } else {
> +        assert(!(ret & BDRV_BLOCK_DATA));
> +        bdrv_aio_discard(s->target, sector_num, op->nb_sectors,
> +                         mirror_write_complete, op);
> +    }
>      return delay_ns;
>  }

Paolo also noticed that there's no reason at all to allocate buffers
and a qiov for the write_zeroes and discard cases.

Kevin