strange performance degrade on 3Ware

[Yakov Hrebtov]

Hello!

I'm building server using 3Ware 9550SX-4LP SATA RAID controller.
OS is FC5.

I have noticed strange performance degrade, when writing to the partition of 
raid device compared to writing to device itself.
Let me explain.

There is only one partition created (/dev/sda1) on my raid device (/dev/sda).
I decided to measure write speed to this partition, using dd command.
dd if=/dev/zero of=/dev/sda1 bs=1M seek=100 count=4096
Writing 4G of zeros to /dev/sda1 takes about 64 seconds, so write speed is 
about 67Mb/sec.
Then I ran this test on raid device itself (/dev/sda):
dd if=/dev/zero of=/dev/sda bs=1M seek=100 count=4096
In this case, dd finishes in 34 seconds, write speed is about 124 M/sec!
(I repeated tests several times in both cases)

What causes so huge difference?

If I do similar write test on regular hard drive attached to this system 
(/dev/sdc), then I have not measured any performance difference in writing to 
/dev/sdc compared with writing to /dev/sdc1.

Kernel version is 2.6.17-1.2187_FC5

I'm ready to provide other info, that you need to help me to solve this.

Thank you!