Odd raid1 rebuild performance

I've been doing some testing lately with raid1 rebuilds.

I'm using an dual PentiumIV Xeon CPU system, with an adaptec UltraSCSI
320 controller and dual 10K RPM SCSI disks.

I simulate a failure of one of the disks, and then simulate the
replacement of the "failed" disk.

The first time I do this, I get good rebuild performance of
approximately 40MB/s.

If I then go ahead and repeat exactly the same test once the rebuild has
completed, I get relatively poor performance of approximately 6.3MB/s. 

If I reboot the system, the rebuild rate goes back up to 40MB/s for the
first rebuild, but the second is poor as above.

There is no change in the system load between the tests. The system load
is virtually non-existent during my testing.

I'm using 2.4.19 kernel, with the 1.3.0 aic79xx adaptec SCSI driver, and
the XFS 1.2 release. I've applied a patch I received from Neil Brown to
reduce the frequency of a kernel oops I was seeing.

Any suggestions as to how I can help to debug this?

Thanks!

Sean.

-- 

Sean C. Kormilo, STORM Software Architect, Nortel Networks
              email: skormilo@nortelnetworks.com