RAIDframe-on-mpt(4) write performance

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

I'm trying to track down abysmally low (15-30MB/s) write performance on a 
file system on a RAIDFframe level 1 RAID on mpt(4) SAS discs.

On a file system on one of the component devices, write performance is 
140MB/s.

On the RAID's raw device, I can write 110MB/s when using 64k blocks.
When I INcrease the block size (like dd bs=128k), the write performance 
DROPs: 48MB/s at 128k, 20MB/s at 256k, 17 at 512, 15 at 1024.

I can't make any sense out of this. Running iostat -x during the tests 
shows 64k (i.e. MAXPHYS) writes on the RAID, 8k (16 SpSU) on the components,
no reads, all as expected. CPUs are close to idle.

On a similar RAIDframe setup, but on SATA discs at on-board controllers, 
write throughput exceeds 100MB/s.

All file systems are 16k fsbsize FFSv2 (i.e newfs -O 2 -b 16k) mounted -o log.

What SAS controllers do other people use? Is mpii(4) ready for production? 
What about SAS hardware RAID controllers in JBOD mode?