Exabyte and file positioning...

[Robert Kuhn]

In article 10203 of this group Robert E. Toense writes:

> There are three possible conditions for writing files on an Exabyte
> drive:
>
> 1) Beginning of tape,
> 2) Immediately following a write (no intervening motion), and
> 3) Immediately following writing a filemark (no intervening motion).
>
> One of these conditions must exist or the write will fail.

This may be true for SOME Exabytes, with SOME drivers, but is by no means
universal.  We have an Exabyte (supplied by Aviv Corp) attached to the
SCSI 3 board of our 4/280 running SunOS 4.0.3.  We perform unattended
backups on this device, a full dump once a week, followed by 6 daily
incrementals.  Since one cannot be assured of tape position being
maintained, we reposition to the beginning of the tape, and then advance
to the correct tape position before each dump.  Here is our Sunday script:

	/usr/bin/mt -f /dev/nrst1 rew
	/usr/bin/sleep 120
	/usr/bin/mt -f /dev/nrst1 fsf 2
	/usr/bin/sleep 120
	/usr/bin/echo "Making a level 6 dump for zariski onto exabyte
	at:" 
	date 
	/etc/dump 6ucfbsdn /dev/nrst1 56 5190 4100000 /dev/rxy0h 
	/usr/bin/echo "Level 6 dump for zariski finished at:" 
	/usr/bin/date 
	/usr/bin/mt -f /dev/nrst1 rew

One problem with the script discussed by Mr Toense:

	tar xf /dev/nrst0
	mt -f /dev/nrst0 rew
	mt -f /dev/nrst0 fsf 1
	dump 4dsbf 54000 6000 100 /dev/nrst0 /dev/...

has been discussed before, namely, the need to pause between operations
that reposition the tape.  Whatever else may be the problem, the simple
prescription for successful tape writing on an Exabyte which he gives is
wrong.

	Yours,

	Robert M. Kuhn
	Director of Computing in Mathematics
	Department of Mathematics
	Harvard University
	1 Oxford Street
	Cambridge, MA 02138