block: blk-merge: fast-clone bio when splitting rw bios

biovecs has become immutable since v3.13, so it isn't necessary
to allocate biovecs for the new cloned bios, then we can save
one extra biovecs allocation/copy, and the allocation is often
not fixed-length and a bit more expensive.

For example, if the 'max_sectors_kb' of null blk's queue is set
as 16(32 sectors) via sysfs just for making more splits, this patch
can increase throught about ~70% in the sequential read test over
null_blk(direct io, bs: 1M).

Cc: Christoph Hellwig <[email protected]>
Cc: Kent Overstreet <[email protected]>
Cc: Ming Lin <[email protected]>
Cc: Dongsu Park <[email protected]>
Signed-off-by: Ming Lei <[email protected]>

This fixes a performance regression introduced by commit 54efd50bfd,
and allows us to take full advantage of the fact that we have immutable
bio_vecs. Hand applied, as it rejected violently with commit
5014c311baa2.

Signed-off-by: Jens Axboe <[email protected]>

0 files changed, 0 insertions, 0 deletions