Why is SCSI so much faster with the write cache off (than ATA)?

[Martin Cracauer]

I have observed it several times and I am missing one bit of knowledge
here: 

why is SCSI so much faster when you turn off the write cache than
P-ATA and SATA?

P-ATA and SATA crumble to about 1/10th of the speed (just writing one
file with 8k blocks linear), whereas SCSI just loses 10-20%, for me.
I have observed that 10 years ago with some 8 GB ATA IBM disk on the
BX chipset versus some 4 GB Quantum Atlas, and now I see it again with
Seagate 7200.7s and .8s versus a 10K Compaq labled 36 GB drive.

Personally I don't see why a linear write should be slow at all.
Surely the computer delivers the data fast enough for sectors to be
filled as they pass under the head.  Maybe the ATA disks lose one
rotation per sector or per filesystem block written anyway? Then why
don't SCSI disks lose the same way given they are not allowed to cache
anything either?

Martin
-- 
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
Martin Cracauer <cracauer at cons.org>   http://www.cons.org/cracauer/
FreeBSD - where you want to go, today.      http://www.freebsd.org/