md/raid5: improve IO pattern

Hi,

These are some patches I'm testing to improve raid5-cache performance. The basic idea is to flush a lot of stripes to raid disks and then do smart scheduling of IOs. In my test of a 12-disk raid6 array, this improves around 20% throughput and request size/disk seek are improved a lot. fio script I'm using are:

[global]
ioengine=libaio
direct=1
loops=1000
runtime=120
time_based=1
file_service_type=random:36
overwrite=1
thread=0
group_reporting=1

[test]
filename=/dev/md0
bs=4k
readwrite=randwrite
numjobs=8
offset_increment=10G

--------------------------------------------
Shaohua Li (3):
md/raid5: prioritize stripes for writeback
md/raid5-cache: bump flush stripe batch size
md/raid5: sort bios

 drivers/md/raid5-cache.c |   2 +-
 drivers/md/raid5.c       | 189 ++++++++++++++++++++++++++++++++++++++---------
 drivers/md/raid5.h       |  13 +++-
 3 files changed, 169 insertions(+), 35 deletions(-)

2.9.3