Promise PDC20267 ATA RAID, poor write performance

Hello,

I have a intel S845WD1-E board with onboard Promise PDC20267 controller.
The system is set up with a P4 2Ghz with 1GB RAM.
The board also has a Intel ICH2, but that's only used by a CDROM.

Disks connected:
ad4: 39205MB <Maxtor 6E040L0/NAR61590> [79656/16/63] at ata2-master 
UDMA100
ad6: 39205MB <Maxtor 6E040L0/NAR61590> [79656/16/63] at ata3-master 
UDMA100

I have them set up in a RAID1

I have tried this on FreeBSD 4.10-RELEASE, and 6-CURRENT (from today) 
with similar results.

I also tried with RAID0, and that showed the same results*2, that is, 
ca. 7MB/s write instead of 3.5MB/s.

I have no previous experience with this hardware on other OS'es, so I'm 
not sure what to expect, but I'm pretty sure it should be better than 
this :)

dmesg included below.

Doing simple tests show very poor write performance:
(reboot)
# dd if=/dev/zero of=fill bs=1m count=2000
2000+0 records in
2000+0 records out
2097152000 bytes transferred in 588.383886 secs (3564258 bytes/sec)

(reboot)
# dd if=fill of=/dev/null bs=1m
2000+0 records in
2000+0 records out
2097152000 bytes transferred in 68.492015 secs (30618927 bytes/sec)


sample output from iostat 10 during write:
    0    8  0.00   0  0.00   0.00   0  0.00  127.18  27  3.39   0  0  3  
1 96
    0    8  0.00   0  0.00   0.00   0  0.00  126.22  27  3.39   0  0  3  
0 97
    0    8  0.00   0  0.00   0.00   0  0.00  122.15  30  3.57   0  0  3  
0 97
    0    8  0.00   0  0.00   0.00   0  0.00  127.59  27  3.41   0  0  3  
0 97

sample output from iostat 10 during read:
    0    8  0.00   0  0.00   0.00   0  0.00  127.53 238 29.61   0  0 14  
1 86
    1   90  0.00   0  0.00   0.00   0  0.00  126.71 241 29.80   1  0 14  
1 84
    0    8  0.00   0  0.00   0.00   0  0.00  127.06 231 28.67   0  0 15  
1 84
    0   62  0.00   0  0.00   0.00   0  0.00  127.52 235 29.26   0  0 14  
1 85

sample output from vmstat -i during write:
interrupt                          total       rate
irq1: atkbd0                           4          0
irq6: fdc0                             5          0
irq8: rtc                          30548        127
irq13: npx0                            1          0
irq14: ata0                           47          0
irq18: fxp0                         3990         16
irq22: atapci0                     18032         75
irq0: clk                          23865         99
Total                              76492        318

sample output from vmstat -i during read:
interrupt                          total       rate
irq1: atkbd0                           1          0
irq6: fdc0                             5          0
irq8: rtc                          21926        127
irq13: npx0                            1          0
irq14: ata0                           47          0
irq18: fxp0                         1523          8
irq22: atapci0                     33047        192
irq0: clk                          17128         99
Total                              73678        428




Mvh,
Frode Nordahl