slowdown on internal harddisks

[Cor Bosman]

Hi all, im running a large usenet setup (30+ servers, 20TB or so) and am
running into some trouble with one of the servers. 

It's a dual Xeon 2.6, 4GB memory, mirrored 2 x 17GB / on ADAPTEC 2015S (asr)
extra 70GB disk on the same asr controller, and 3 x 1.5 TB on 
Qlogic ISP 2300 PCI FC-AL Adapter (isp) set up as 6 750GB partitions. 

This server pushes out about 300mbit of data on an Intel Pro 1000 (em).
Im also using a 1GB MFS which i need to support a very high amount of seeks
on a usenet history file (300 MB used).

As the server starts doing lots of io on the Qlogic controller all disk writes
seem to drop to about 0. Also on the MFS, and on the internal asr controller.
Copying a file from any disk to any other, even to the unused extra 70GB
disk has a max throughput of about 50KB/s. Read performance is still ok, it's
just write()s that are slow. (cp-ing to /dev/null is fast).

Ive tried all kinds of tweaks but cant seem to find the cause of this.
It's especially weird because another, identical, server, with the same load
does not show this problem. 

Any ideas?

Regards,

Cor