Geoff -

Do a physical inspection of the D40s, in particular looking for
blinking lights indicating incomplete SSA loops which degrade
performance. Your AIX Error Log may also hold indications of problems.

All the usual system tuning considerations need to be pursued. One of
the first things I would look at is memory and paging space. If you
are running a TSM client on that TSM server, use dsmcad rather than
dsmc schedule, to avoid tying up vital memory in perpetuity. I would
also conduct relative performance measurements, comparing I/O
performance for a given write load to the external SSA vs. the same
to internal disk.

In not using RAID for your D40 disks, you're giving up a great
opportunity for performance. I'm using RAID 1+0 on D40 SSA and am
enjoying great performance. Make the most of the available technology.

    Richard Sims