386GT Adaptec SCSI performance :-(

[neese at adaptex.UUCP]

>>Anybody have any ideas?
>
>CDC by default ships the Wren IV drives with the read ahead buffer disabled.
>If you enable this via the mode sense/select commands, then you will see
>about a 400% increase in throughput.

You can also patch the kernel SCSI driver to increase the speed of the
DMA on the 1542A.  The default that SCO chose to use was 5MBytes/sec.
This is due to the fact that some motherboards will not run any faster
than that but most will run faster than that.  The Tandy 4000 will run
at 6.7MBytes/sec and the Tandy 4000LX will run at 8MBytes/sec.
The value that needs to be changed in the 2.3GT kernel is ad_xfer.
It is currently set to zero.  The following table summarizes the value
to the transfer rate.

0	5 MBytes/sec
1	6.7 MBytes/sec
2	8 MBytes/sec
3	10 MBytes/sec
4	5.7 MBytes/sec

You need to use adb and patch the data arear at ad_xfer.  Keep a good
kernal around while you experiment.  If the speed is too fast then
you will get a parity interrupt panic as soon the the driver attempts
to run the ad_init code.  Nothing on the drive will be hurt, but you
will need a good kernel to boot from.  If possible the optimum speed
should be set at 6.7MBytes/sec.  This allows full SCSI bandwidth
transfers in synchronous mode.  Setting it much faster than that will
cause the host adapter to starve the buffer on the drive and cause
multiple disconnects during data transfers, even small ones such as
1K.  Play around and do what is best for your system.

			Roy Neese
			Adaptec Central Field Applications Engineer
			UUCP @ {merch,texbell,killer}!cpe!adaptex!neese