(S)ATA performance in FBSD 6.2/7.0

[O. Hartmann]

The last days I tried to figure out why some of my lab's FreeBSD boxes 
and also mine at home seem to be outperformed by some Linux setups 
around here and I saw something interesting.

On my lab's FreeBSD 6.2/i386 box (ASUS P4P800, ICH5 with two SATA 150 
ports, two SATA 300 drives attached) I copied big files (~ 5GB) from one 
drive to another while the box didn't do anything else than copying. I 
watched the copy process via 'systat -vmstat 1' and realized, that the 
value of 'KB/t' never go byond 128 (128kb buffer limit?). But more 
frustrating, I never got beyond 33 MB/s transfer rate although 
bonni/bonni++ told me both drives are capable doing much more (~75 MB/s 
each).
At home, I use a FreeBSD 7.0-CURRENT box on an ASUS 
A8N32-SLI/nForce4-SLI based box, amd64 (no 32Bit compatibility). Two 
Hitachi T7K250 250 GB/SATA II drives build up a RAID 0 (nVidia 
MediaShield), and additionally there is a SAMSUNG Spinpoitn SP2004C 
attached to the controller. bonni results in 55 MB/s for the SP2004C 
alone and gives ~ 65 - 70 MB/s for the Hitachis, each and roughly 115 
MB/s for the RAID 0. But copying from the single drive to the RAID 0 or 
from the RAID 0 to the single drive also reaches this oscure 33 MB/s 
boundary!

In the first place I thought the older i386 hardware has some 
hard-limits, but we have several boxes of the exact same hardware around 
here and a wide spread Linux and Windows utilization and on those boxes 
  equipted with more than one harddrive (PATA or SATA) the effective 
transfer rate shown up is about 50 - 65 MB/s as expected with copying a 
big 5G file from one drive to another.

The hardwrae limit is completely nonsense when it comes to the AMD64 box 
with newer hardware.

Before digging into this problem deeper with benchmarks, could anyone 
explain why FreeBSD reaches this 33 MB/s limit (sounds like UDMA 33 
defaults, but on both boxes nForce4 and ICH5 controller are recognized 
and show up with SATA300 or SATA150 capabilities, respective)? May I 
have some knobs I'm not aware of to tune disk performance?

I would appreciate any coments on that and if someone has some good 
ideas how to benchmark those subjects, please let me know.

Regards,
Oliver
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