SAS drives seem slow

[Josh Paetzel]

I have a 3ware 9690SA SAS RAID controller in a PCI-e 8x slot with  
Fujitsu MBA series 15k SAS drives attached, and the array is coming up  
as:

da0 at twa0 bus 0 target 0 lun 0
da0: <AMCC 9690SA-4I  DISK 4.06> Fixed Direct Access SCSI-5 device
da0: 100.000MB/s transfers
da0: 138272MB (283181056 512 byte sectors: 255H 63S/T 17627C)

The controller is probed as:

twa0: <3ware 9000 series Storage Controller> port 0x2000-0x20ff mem  
0xe0000000-0xe1ffffff,0xe4100000-0xe4100fff irq 16 at device 0.0 on pci2
twa0: [ITHREAD]
twa0: INFO: (0x15: 0x1300): Controller details:: Model 9690SA-4I, 128  
ports, Firmware FH9X 4.06.00.004, BIOS BE9X 4.05.00.015

FreeBSD services.tcbug.org 8.0-CURRENT FreeBSD 8.0-CURRENT #0: Mon Feb  
16 21:07:14 UTC 2009     root@:/usr/obj/usr/src/sys/SERVICES  amd64

But 6.x and 7.x give similar results.

The disks themselves are capable of sequential read/write in the 180  
Meg/sec range, so I'm trying to understand why I'm being told they are  
100 Meg/sec, and why that seems to be their real world performance cap.

# dd if=/dev/zero of=stuff bs=8m count=100
100+0 records in
100+0 records out
838860800 bytes transferred in 8.711125 secs (96297644 bytes/sec)

Thanks,

Josh Paetzel