Re: [PATCH v2 1/7] block/block-copy: specialcase first copy_range request

[Max Reitz <mreitz@redhat.com>]

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On 27.11.19 19:08, Vladimir Sementsov-Ogievskiy wrote:
> In block_copy_do_copy we fallback to read+write if copy_range failed.
> In this case copy_size is larger than defined for buffered IO, and
> there is corresponding commit. Still, backup copies data cluster by
> cluster, and most of requests are limited to one cluster anyway, so the
> only source of this one bad-limited request is copy-before-write
> operation.
> 
> Further patch will move backup to use block_copy directly, than for
> cases where copy_range is not supported, first request will be
> oversized in each backup. It's not good, let's change it now.
> 
> Fix is simple: just limit first copy_range request like buffer-based
> request. If it succeed, set larger copy_range limit.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>  block/block-copy.c | 41 ++++++++++++++++++++++++++++++-----------
>  1 file changed, 30 insertions(+), 11 deletions(-)
> 
> diff --git a/block/block-copy.c b/block/block-copy.c
> index 79798a1567..8602e2cae7 100644
> --- a/block/block-copy.c
> +++ b/block/block-copy.c

[...]

> @@ -168,7 +170,21 @@ static int coroutine_fn block_copy_do_copy(BlockCopyState *s,
>              s->use_copy_range = false;
>              s->copy_size = MAX(s->cluster_size, BLOCK_COPY_MAX_BUFFER);
>              /* Fallback to read+write with allocated buffer */
> -        } else {
> +        } else if (s->use_copy_range) {
> +            /*
> +             * Successful copy-range. Now increase copy_size.
> +             * copy_range does not respect max_transfer (it's a TODO), so we
> +             * factor that in here.
> +             *
> +             * Note: we double-check s->use_copy_range for the case when
> +             * parallel block-copy request unset it during previous
> +             * bdrv_co_copy_range call.

But we should still goto out anyway, shouldn’t we?

> +             */
> +            s->copy_size =
> +                    MIN(MAX(s->cluster_size, BLOCK_COPY_MAX_COPY_RANGE),
> +                        QEMU_ALIGN_DOWN(block_copy_max_transfer(s->source,
> +                                                                s->target),
> +                                        s->cluster_size));
>              goto out;
>          }
>      }
> @@ -176,7 +192,10 @@ static int coroutine_fn block_copy_do_copy(BlockCopyState *s,
>      /*
>       * In case of failed copy_range request above, we may proceed with buffered
>       * request larger than BLOCK_COPY_MAX_BUFFER. Still, further requests will
> -     * be properly limited, so don't care too much.
> +     * be properly limited, so don't care too much. Moreover the most possible

s/possible/likely/

Max

> +     * case (copy_range is unsupported for the configuration, so the very first
> +     * copy_range request fails) is handled by setting large copy_size only
> +     * after first successful copy_range.
>       */
>  
>      bounce_buffer = qemu_blockalign(s->source->bs, nbytes);
> 



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