Description of problem:

LSI SAS 9300-8i write performance too slow
(whilst read performance is just fine).

Version-Release number of selected component (if applicable):

Kernel 3.12.9-201.fc19.x86_64


How reproducible:
Always


Steps to Reproduce:
1. attach 3 SSDs to adapter
2. create striped mapped device (e.g. using LVM)
3. dd of=/dev/$TheStripedDevice oflag=direct if=/dev/zero iflag=fullblock bs=2G count=1

Actual results:

Single threaded read performance is as expected, ie. 3 SSDs (3 * Samsung 840EVO  1TB) with per drive sustained read throughput of 540MB/s lead to ~1.6GB/s on a striped logical volume (all 3 attached to one SFF-8643 port of the HBA via a 1:4 mSAS to SAS cable).

Write performance on the same striped LV as in "Steps to Reproduce" (1 thread) maxes out at ~750MB/s as opposed to the maximum write throughput of ~490MB/s on each of the 3 SSDs written one by one

Measured:
- ~490MB/s writing (1 thread) to the individual SSDs one by one
- ~750MB/s writing (1 thread) to all 3 SSDs in parallel via the striped LV
- ~1GB/s (3 * ~340MB/s) writing (3 threads) to all 3 SSDs in parallel;
     ie. better than the striped LV write performance


Expected results:

Expecting >1.4GB/s write maximum on the striped LV written single threaded.