ADSM treats standard SCSI DASD very differently than an optical library.
The DASD is accessed in very random mode, and attempts to fill up multiple
DASD disks equally.  On the other hand optical media is treated in a
very sequential manner for better streaming performance.

For more efficient use, ADSM would be better off using the optical library
as an optical library and not a large SCSI disk.  Treating a large library
as a disk may result in having the library thrash (mount disk a, write
4 blocks, mount disk b, write 6 blocks, mount a again, etc.).

On the other hand ADSM will try to minimize mounts of optical disks if used
as a library.  Once a disk is mounted data is written until no further
room is available or no further data is available.  Same for reads.  Disks
are mounted in efficient order to stream reads.  Doing random reads (when
configured as a single big disk) may lead to multiple mounts of the same disk.

For optical disks the performance penalty exists in the initial mount.
At that point reads and writes are fairly efficient, though nowhere as
fast as your 3490's.  For 3490's, you have mount time and seek time before
data is moved.  If it is small data optical is faster.  If it is big data
3490 is faster.

I hope this helps you configure your system.

Paul