On Thu, Nov 22, 2018 at 06:26:17PM +0800, Ming Lei wrote:

Suppose one bio includes (pg0, 0, 512) and (pg1, 512, 512):

The split is introduced by the following code in blk_bio_segment_split():

      if (bvprvp && bvec_gap_to_prev(q, bvprvp, bv.bv_offset))
		  	goto split;

Without this patch, for non-cluster, the two bvecs are just in different
segment, but still handled by one same bio. Now you convert into two bios.

Oh, true - we create new bios instead of new segments.  So I guess we
need to do something special here if we don't want to pay that overhead.

Or just look into killing the cluster setting..