NetNews Usenet Archive 1993 #3/NN_1993_3.iso/spool/comp/os/vms/21953

Internet Message Format | 1993-01-25 | 2.0 KB

Path: sparky!uunet!olivea!spool.mu.edu!agate!ucbvax!galaxy.dnet!gleeve
From: [e-mail]
Newsgroups: comp.os.vms
Subject: Exabyte tapes
Message-ID: <[e-mail]>
Date: 25 Jan 93 13:36:14 GMT
Sender: [e-mail]
Organization: The Internet
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At the DECUS symposium in Las Vegas, there was an Exabyte 8500
drive on the floor, the only 8mm there, on a VAXstation. thus
it was using MKdriver, a version that has no density control. Since
I couldn't get good information there about how to set one or more
of the slide switches on the back of the drive to write low
density, I was able to get the thing to write low density (EXB 8200
series compatible) tape by using back/list mka400:* to space past
the stuff on a previously init'ed 8mm, at low density, and then
just writing with backup. The resulting tapes could be read on
an 8200, though the process was somewhat flaky (some probability of
parity errors at the "boundary"). I am assured that the right switches
(in conjunction with power cycling, SCSI resets,and other incantations)
can get the EXB 8500 drives to write low density. On another drive
in Raxco's Rockville office, I regularly write as well as read
8200 compatible tapes on an EXB 8500 drive; that has an HSC
interface, so I just say density=1600 for low density. The tapes
are not preformatted.
   With small enough blocks and slow data feeds you reduce capacity.
However, EXB end of file marks are very long; if you are trying
to write lots of files, delimited by EOF marks, on the tape (e.g.
by using it as an ANSI volume, which some controller/driver combos
will let work), you can easily bring the data capacity down to, or
even far below, 500 MB. Just recall how long it takes a video tape
to synchronize picture and sound when you overwrite starting at
the middle...the 8mm drive has to have a long enough EOF to recognize
in spite of a similar low level problem. It works, but occupies a very
noticeable amount of tape (and accounts for the slow write-eof
operation).
Glenn
[e-mail]