exabyte record size limit

[Mark Bartelt]

I have an exabyte drive (from Dilog) on a 4D25.  When I recently upgraded
to IRIX 3.3.1, I was pleased to find the /dev/{r}mt/tps*v devices:  It's
nice to finally be able to read/write arbitrary-length tape records!

However, I've found that there's a record size limit of 240k bytes.  Any
attempts to write records longer than that return EINVAL.  Is this just a
misfeature of the IRIX tape driver, or is it a characteristic of exabyte
drives, or a limit (general, or SGI's) on the size of SCSI transfers, or
what?  And, whichever, why was 240k chosen?

For that matter, should I even care?  Back in ancient times, when all we
had were half-inch drives at, say, 800 or 1600 bpi, it was worth writing
data in blocks as large as possible, to minimize the amount of tape that
was wasted in inter-record gaps.  How, exactly, do exabytes separate the
physical records?  And how much tape does that information use, compared
with the length of an N-byte record?

Mark Bartelt                                                416/978-5619
Canadian Institute for                          sysmark at cita.toronto.edu
Theoretical Astrophysics                        sysmark at cita.utoronto.ca