Since you don't want to write any one tape at more than one buffer size, 
it's good practice to suspend "Active" tapes (not full and not empty) 
when you make the change.  There's a tech note on this someplace.

cheers, wayne

Carlos Alberto Lima dos Santos wrote, in part,  on 6/22/2005 12:45 PM:

>       Also check the /kernel/drv/st.conf file and verify the entries for
>the tape driver are correct.   
>       
>Symptom: 
>How to optimally performance-tune systems with IBM Linear Tape-Open 2 drives
>(Linear Tape-Open generation 2 drives, IBM 3580 Ultrium-2) 
> 
>Solution: 
>
>1. Hardware vendors suggest a 256 KB block size for maximum I/O to the IBM
>Linear Tape-Open (LTO) generation 2 tape head. 
>
>   To do this, create the following file:
>     /usr/openv/netbackup/db/config/SIZE_DATA_BUFFERS
>   and add this line:
>     "262144."
>
>2. ...
>  
>