X25-e / adaptec 5805 /raid0 throughput problem

For my application (data acquisition) I need stream as fast as possible to disk

Inspired by the posted 2.2GB/s results posted in Tom's hardware, I set up a RAID0-array using 8x Intels X25-E ssd drive (32GB) and an adaptec 5805 RAID controller, hoping to achieve about half the speed (1.1GB/s) when using half the number of drives (8 vs. 16). Even though I need the storage for sequential writing I went with SSD technology for lower power consumption / no cooling issues.

Unfortunately the sequential writing throughput is by far lower than expected. When streaming to a single disk, I get an average of 196 MB/s writing speed, with a short peak of ~ 400MB/s at start (I assume this is fill-up of some cache). Using a RAID0-array of two disks I get 382MB/s, which seems to me correct scaling.

However if I try to add more disks to my RAID-array I am stuck at ~400MB/s.
What can be the cause of that?

Tools used: HD Tune and h2benchw (pretty much consistent in results)

System:
ASUS P6T6 WS Revolution
12GB memory
Intel i7 920 processor @ 2.67GHz
NVIDIA Quador FX 370
Microsoft Windows Vista 64bit


My guess is that there is some bottleneck either in the RAID controller or in the 8-lane PCI-e bus ???
I tried to switch PCI-e slots, checked all drives individually, played with cache settings at the controller and used different stripe sizes. I do not see any PCI-e settings in the motherboard BIOS. Only two PCI-e slots are occupied, one by the graphics card, one by the RAID controller.

Thanks for any hints where to look or what further tests I could do!

Ingmar.

Even by using both ports I only get 400MB/s (7-disk RAID 0). I the meantime I got some slight improvement when using the newest Adaptec Vista drivers and the newest Adaptec BIOS.
Now the average write speed to a 7-disk (X25-E) RAID-0 array is 465.7MB/s,
The write speed to a 3-disk RAID-0 array (using the same port on the adaptec controller) is 456.5 MB/s, for two disks I get 385.7MB/s average.
So using more than 3 disks and using more than 1 port on the controller gives no significant improvement in throughput. I am still puzzled.