Re: [PATCH] md/raid5: set chunk_sectors to enable full stripe I/O splitting

[Paul Menzel]

Dear Kuai,


Thank you for your patch.

Am 23.02.26 um 04:58 schrieb Yu Kuai:
> Set chunk_sectors to the full stripe width (io_opt) so that the block
> layer splits I/O at full stripe boundaries. This ensures that large
> writes are aligned to full stripes, avoiding the read-modify-write
> overhead that occurs with partial stripe writes in RAID-5/6.
>
> When chunk_sectors is set, the block layer's bio splitting logic in
> get_max_io_size() uses blk_boundary_sectors_left() to limit I/O size
> to the boundary. This naturally aligns split bios to full stripe
> boundaries, enabling more efficient full stripe writes.
>
> Test results with 24-disk RAID5 (chunk_size=64k):
>    dd if=/dev/zero of=/dev/md0 bs=10M oflag=direct
>    Before: 461 MB/s
>    After:  520 MB/s  (+12.8%)

Sweet.

> Suggested-by: Christoph Hellwig <hch@xxxxxxxxxxxxx>
> Signed-off-by: Yu Kuai <yukuai@xxxxxxxxx>

Once committed, should this be backported to the longterm and stable series?

> ---
>   drivers/md/raid5.c | 1 +
>   1 file changed, 1 insertion(+)
>
> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
> index 8854e024f311..810d936560d1 100644
> --- a/drivers/md/raid5.c
> +++ b/drivers/md/raid5.c
> @@ -7783,6 +7783,7 @@ static int raid5_set_limits(struct mddev *mddev)
>   	lim.logical_block_size = mddev->logical_block_size;
>   	lim.io_min = mddev->chunk_sectors << 9;
>   	lim.io_opt = lim.io_min * (conf->raid_disks - conf->max_degraded);
> +	lim.chunk_sectors = lim.io_opt >> 9;
>   	lim.features |= BLK_FEAT_RAID_PARTIAL_STRIPES_EXPENSIVE;
>   	lim.discard_granularity = stripe;
>   	lim.max_write_zeroes_sectors = 0;

Reviewed-by: Paul Menzel <pmenzel@xxxxxxxxxxxxx>


Kind regards,

Paul