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From: rlr@alumni.colorado.edu (Roger Rose)
Newsgroups: comp.periphs.scsi
Subject: Re: High Capacity Tapes: Exabyte or DAT?
Message-ID: <1991May28.204639.6253@colorado.edu>
Date: 28 May 91 20:46:39 GMT
References: <29543@hydra.gatech.EDU> <9850023@hpcpbla.HP.COM> <1991May28.130950@anusf.anu.edu.au>
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> One of our vendors has told me that the 5GByte figure on the capacity
> on the newer Exabyte drives is achived by on board data compression and
> that the actual capacity depends on the compressability of your data.

Well, your vendor is simply wrong. ;-)  There are 3-rd party compressed
versions of the EXB-8200 (2.4Gb) drive available; however, the EXB-8500
(5Gb) drive is not compressed.  The capacity gain was accomplished by
doubling the track density.  A compressed version of the drive has also
been announced, but the compression will be in addition to the
**native** 5Gb capacity.

> So presumably the actual capacity of a tape is anywhere from 2.3GBytes
> to 5GBytes. Is this true? If it is, does anyone have any figures on
> what the "typical" capacity is when backing up a "typical" filesystem?

This depends on a few factors.  Two major factors being how good the
error statistics are on the tape and how well you keep the drive
streaming.  The stated capacity is the formatted physical capacity of
the drive.  (You can get the exact figures to logical-end-of-tape from
the manuals.  The numbers to physical-end-of-tape vary slightly from
tape to tape.)

Tape quality affects capacity, because questionable blocks are rewritten
and this consumes space.  Actual loss on data-grade tapes should run
less than 1% even after the tapes have been used several times.

Streaming affects capacity, because the drive attempts to maintain
streaming by inserting gaps into the data.  In order to stream an
EXB-8500, you need to maintain a 500Kb **average** data rate from the
host.  If this data rate is not maintained, then gaps are inserted
whenever the drive's cache empties.  (There are ways of tuning the
drive to minimize the impact of transfer rates, if your driver software
knows how.)

-- 

Roger Rose  {rlr@boulder.colorado.edu}