Bear in mind that you may not be able to raise the buffer size above 64K 
unless your driver allows it.  That depends on how the drive is connected 
and how the driver works.

For example, the MS SCSI port driver is limited to 64K unless you set the 
MaximumSGList parameter.  If you do a Google search for that, you can find 
out more.  There is an HP paper on how to tune their LTO drive 
performance, which pretty much applies to any drive.

If you use FC interface, it will depend on your HBA driver.   Compaq use 
the MS port driver and their own miniport driver, so you can set the 
MaximumSGlist.  Emulex use their own port driver that completely replaces 
the MS driver, so there is no such parameter - but you can set large 
buffer sizes anyhow.

What is best for you will depend on all sorts of things.  Get the Veritas 
paper on tuning on Windows and work through it.  You may find that the 
tape is not the slow bit...and that you have to multiplex to keep it 
streaming.   Just study the 'wait & delay' timers, you'll soon see if you 
are keeping the drive streaming.

The other thing to look at is the data - file size, count and 
compressibility. That way you can test with known files, and try to 
extrapolate to your real data.   The Veritas tool that is for the Storage 
Migrator will do nice profiles of file size on any file system.   For 
compressibility you can try just ZIP'ing a slab of data.  If it is very 
compressible, you will have to get data to the tape much faster to keep it 
going.  For example, if the data is 2:1 compressible, and you have 64K 
buffers, you must get data coming in at 30MB/s - that is 480 
buffers/second - so you may need a really big pile of buffers and 
multiplexing.

BTW HP also have some tools on their web site to create standard data sets 
with different compressibility.  ZIP does not agree with how compressible 
the sets are, but it's useful.  They also have tools to test tape 
throughput, but I've not looked to see if thee only work on HP drives.

William D L Brown