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

[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%)

Suggested-by: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Signed-off-by: Yu Kuai <yukuai@xxxxxxxxx>
---
 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;
-- 
2.51.0