lukem@cs.rmit.edu.au (Luke Mewburn) writes:
>I'd be very surprised if you got more than 1.3 - 1.5MB/s through an
>AHA1542B (or any other ISA SCSI controller), no matter how fast the
>drive is (that's been my experience anyway).

dd'ing 1024 128k blocks from the raw device I get

Quantum PD425S	1680 KB/s
Micropolis 1684	1424 KB/s
IBM DCAS-34330	3048 KB/s

through a BT545S on my 486/133.  CPU usage was about 5% for the first two
and about 10% for the last disk.  It seems that this controller is limited
to somewhere around 3MB/s aggregate throughput.  I get that in total
when I run the dd's in parallel.  It certainly isn't limited by the number
of I/O requests per second because I can get it up to about 240 IO/s for
all three disks together with smaller block sizes.

Also, those 3048 KB/s match pretty good what I'd expect from the 5.0 MB/s
DMA rate and the 7:4 duty cycle on the bus that the BT545 is configured
to.  (And I do know boards that can have the DMA rate set to 6.7 MB/s, so 
4MB/s is doable given good hardware.)

-- 
Christoph Badura
Verlag O'Reilly