mt bsf with sequential dumps to Exabyte

[Stuart Levy]

Appending to Exabyte tapes is a pain with the Sun 4.1 driver, but
possible.  You have to accomplish the following:

     - position the tape immediately PRECEDING an EOF mark
	(preferably the one following the last useful data on the tape :->)
     - [re]write that EOF

After this it's safe to write data, possibly multiple files, until you
reposition the tape (rewind, skip files &c.).  Supposing there were <N>
data files already on the tape, with the last one followed by a double
EOF, normal UNIX tape drivers would perform the above sequence with

	mt rew
	mt fsf <N>	# Skip past last filemark, between double EOFs
	mt bsf 1	# Position after last data, before double EOF
	mt weof		# Rewrite EOF

However, Sun's tape driver is brain-damaged and does not allow this; it
doesn't have the idea of the beginning-of-tape side of a file mark.  But
you can get the desired effect with

	mt rew
	mt fsf <N-1>	# Skip to BEGINNING of last file
	dd if=/dev/nrst0 bs=63k of=/dev/null
			# Read last file's data
			# The tape is now positioned after last data
	mt weof		# Rewrite EOF

This seems to work at least with the Sun 4.1 SCSI driver.  

The above owes much to George Goble of Purdue, who wrote a fine
explanation of the Exabyte's workings and the Sun SCSI tape driver's
peculiarities; it appeared in Sun-spots around the turn of 1988-89 I
think.

[Ed's Note: The articles are in v7n46 and v7n68, circa Dec 1988. -bdg]]

	Stuart Levy, Geometry Group, University of Minnesota
	slevy at geom.umn.edu