Just as a "by the way", I *think* the reason we (BSDI) went
with 12/10/10 (major/unit/subunit), rather than, say, 12/12/8,
was specifically to be able to get the 8 bits of SCSI tape
density code in as a "subunit" and still have the two flag
bits we needed for fixed-vs-variable records and no-rewind.

This was all rather a while ago, though, so it is hard to
remember.

Also incidentally, my ulterior motive for giving out all this
information is to make it easier for BSDI to steal from NetBSD
drivers in the future. :-)  (Well, also to make it easier for
third-party vendors to share their driver code across multiple
BSDs, for instance.)

Chris