(from the Windows Administrator's Guide, page 118)

Increased Block Size for Writing to Tape

Tivoli Storage Manager provides a utility, DSMMAXSG, that can improve
the rate at which the server processes data for backups and restores and
for archives and retrieves. Actual results will depend upon your system
environment. The utility does not affect the generation of backupsets. 
The utility increases the maximum transfer length for some Host Bus
Adapters (HBAs) and, consequently, the block size used by the Tivoli
Storage Manager server for writing data to and getting data from the
following types of tape drives:

--3570
--3590
--3592
--DLT
--DTF
--ECARTRIDGE
--LTO

The maximum supported block size with this utility is 256 KB. When you
run DSMMAXSG, it modifies one registry key for every HBA driver on your
system. The name of the key is MaximumSGList. 
Normally, the utility is executed automatically as part of the Tivoli
Storage Manager server or storage agent installation. However, if you
install a new HBA on your system after server or storage agent
installation or if you install a new version of an existing HBA device
driver that resets the value of the maximum transfer size, you must run
the utility manually in order to take advantage of the larger block
size.

Note: If you back up or archive to tape using the 256 KB block size, you
cannot append to or read from the tape using an HBA that does not
support the 256 KB block size. For example, if you use a 256 KB Windows
system for backing up client data to the Tivoli Storage Manager server,
you cannot restore the data using a Windows system that supports a
different transfer length. If you want to append to or read from tape
written to using a 256 KB transfer length, you need to install an HBA
that supports 256 KB transfers.

For more information on the DSMMAXSG utility, see the Administrator's
Reference.

--
Mark Stapleton (stapleton AT berbee DOT com)
Office 262.521.5627