Very slow disk writes because of wdrain

Hello,

I've upgraded a Server of mine to FreeBSD-6.0-BETA3 today (fresh
install), and I've run into a problem. Writes to the disk are extremely
slow, eg:

  $ dd if=/dev/zero of=/usr/test bs=1048576 count=100
  100+0 records in
  100+0 records out
  104857600 bytes transferred in 13.771165 secs (7614287 bytes/sec)
  $

During the write, dd spends pretty much all it's time in the wdrain
state, CPU usage is about 5% (this is a Pentium 4 2.4GHz). Reading from
the disk (as far as I can tell by dd'ing stuff to /dev/null) is as fast
as it's supposed to be.

wdrain is defined in /usr/src/sys/kern/vfs_bio.c:384.

A posting to questions in march last year recommends increasing
vfs.hirunningspace (default: 1M) to 4M. This has no impact however on
the speed, nor does increasing it to 16M.

The disk in question is a SCSI RAID 1 array, which has been working very
well under FreeBSD 4.11 before. I'm running a custom kernel with the
debugging options as well as the hardware I don't use removed. Please
see the attached dmesg.txt and kernel.txt for details.

Now, I'd like to get fast speeds again from this machine, can anyone
help me?

Thanks,
Benjamin