Block Size

Hi, I try to change the block size from 512 to 0, but it send this message:
0514-068 Cause not know
Can someone help me whith this?

y are you changing the block size to zero?

Tape drive?
Block size = 0 means variable / OS default.

What type of drive / device?
Do you have the correct device driver filesets loaded?

This is the command that I used
chdev -l rmt0 -a blocksize=0
thanks...