Create a raid5 using four disk and the chunksize is 512K.
Test command is: dd if=/dev/zero of=/dev/md0 bs=1536K count=90000 oflag=direct

In RHEL6(kernel 2.6.32):speed about 240MB/s
In 3.5.0-rc5:speed about 77MB/S
Add two patch in 3.5.0-rc5, speed about 200MB/S.

So the performance of odirect-wrirte for block-deivce was obvious reduced.
PATCH 1/2: Add blk_plug function for odirect-write block-device
PATCH 2/2: Remove REQ_SYNC for odirect-write in raid456.

PATCH 2/2 maybe not correct because it alse for odirect-write for regular file.
Jianpeng Ma (2):
  fs/block-dev.c:fix performance regression in O_DIRECT writes to    
    md block devices.
  raid5: For write performance, remove REQ_SYNC when write was odirect.

 drivers/md/raid5.c |    3 +++
 fs/block_dev.c     |    7 ++++++-
 2 files changed, 9 insertions(+), 1 deletions(-)

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1.7.5.4