Tejun Heo wrote:

> Everything seems fine in the dmesg.  Performance degradation is
> probably some other issue in -rc kernel.  I'm suspecting recently
> fixed block layer bug.  If it's still the same in the next -rc,
> please report.

In fact, it's CFQ. The PATA thing was a red herring. 2.6.20-rc2 and 3 
give me ~ 24 MB/s from "hdparm t /dev/hda" while 2.6.20-rc1 and below 
give me ~ 50 MB/s.

Jens: this is due to "[PATCH] cfq-iosched: tighten allow merge 
criteria", 719d34027e1a186e46a3952e8a24bf91ecc33837:

http://www2.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=719d34027e1a186e46a3952e8a24bf91ecc33837

If I revert that one, I have my 50 M/s back. config and dmesg attached 
in case they're useful.

Rene