Commit e36f6204 authored Jan 12, 2016 by Keith Busch Committed by Jens Axboe Jan 12, 2016

block: split bios to max possible length



This splits bio in the middle of a vector to form the largest possible
bio at the h/w's desired alignment, and guarantees the bio being split
will have some data.

The criteria for splitting is changed from the max sectors to the h/w's
optimal sector alignment if it is provided. For h/w that advertise their
block storage's underlying chunk size, it's a big performance win to not
submit commands that cross them. If sector alignment is not provided,
this patch uses the max sectors as before.

This addresses the performance issue commit d3805611 attempted to
fix, but was reverted due to splitting logic error.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Cc: Jens Axboe <axboe@fb.com>
Cc: Ming Lei <tom.leiming@gmail.com>
Cc: Kent Overstreet <kent.overstreet@gmail.com>
Cc: <stable@vger.kernel.org> # 4.4.x-
Signed-off-by: Jens Axboe <axboe@fb.com>

parent cda22646

block/blk-merge.c

+16 −3

Original line number	Diff line number	Diff line
		@@ -83,9 +83,6 @@ static struct bio blk_bio_segment_split(struct request_queue q,
		struct bio *new = NULL;

		bio_for_each_segment(bv, bio, iter) {
		if (sectors + (bv.bv_len >> 9) > queue_max_sectors(q))
		goto split;

		/*
		* If the queue doesn't support SG gaps and adding this
		* offset would create a gap, disallow it.
		@@ -93,6 +90,22 @@ static struct bio blk_bio_segment_split(struct request_queue q,
		if (bvprvp && bvec_gap_to_prev(q, bvprvp, bv.bv_offset))
		goto split;

		if (sectors + (bv.bv_len >> 9) >
		blk_max_size_offset(q, bio->bi_iter.bi_sector)) {
		/*
		* Consider this a new segment if we're splitting in
		* the middle of this vector.
		*/
		if (nsegs < queue_max_segments(q) &&
		sectors < blk_max_size_offset(q,
		bio->bi_iter.bi_sector)) {
		nsegs++;
		sectors = blk_max_size_offset(q,
		bio->bi_iter.bi_sector);
		}
		goto split;
		}

		if (bvprvp && blk_queue_cluster(q)) {
		if (seg_size + bv.bv_len > queue_max_segment_size(q))
		goto new_segment;

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