Tape blocksize for 8mm

[Scott Holt]

We have a number of IBM 8mm drives, both internal an external, 
connected to various RS 6000 systems. I have come across a 
problem using backup with the devices. In particular, if the
block size device attribue is set to 0 (indicating a variable 
length block), tapes subsequently written with backup cannot be
read.

I have poured through various manuals and found no mention of
the range of legal block sizes for the 8mm devices. The system
does not issue any kind of message when the block size attribute
is set to zero. To confuse things even more, other commands 
(such as tar and dd) access the drive fine with the blocksize
set to 0, only backup/restore seem to dislike it.

So, the query is

1) Is backup broke?
2) Is setting blocksize to zero on the 8mm device a dumb idea?
   If so, why does the system let you do it?
3) Where might the legal attribute values for particular devices
   be documented?

- Scott