Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!nstn.ns.ca!news.cs.indiana.edu!mips!zaphod.mps.ohio-state.edu!swrinde!elroy.jpl.nasa.gov!ncar!hao.hao.ucar.edu!bovet
From: bovet@hao.hao.ucar.edu (Ray Bovet)
Newsgroups: comp.periphs.scsi
Subject: Re: High Capacity Tapes: Exabyte or DAT?
Message-ID: <11611@ncar.ucar.edu>
Date: 31 May 91 16:16:20 GMT
References: <29543@hydra.gatech.EDU> <9850023@hpcpbla.HP.COM> <1991May28.130950@anusf.anu.edu.au>
Sender: news@ncar.ucar.edu
Organization: High Alititude Observatory/NCAR, Boulder CO
Lines: 32

In article <1991May28.130950@anusf.anu.edu.au> mbl900@anusf.anu.edu.au (Mathew BM LIM) writes:
>I am new to this discussion so I may have missed the answer to the following
>question.
>
>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. 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?
>



There are two major flavors of Exabyte drives.  The original drives are
the model 8200.  These drives hold up to 2.3 Gbytes on a standard 106 m
(P120) 8mm video tape.  The "new" Exabyte drive is the EXB8500.  This
drive doubles the track density so that a 106 m tape can hold about 4.6 Gbytes.
Exabyte claims 5 Gbytes on the 8500 because they now sell 112 meter tapes
for it.  All of these numbers are without any compression.

Several vendors of Exabytes have incorporated their own compression schemes
external to the drive.  In addition, Exabyte is committed to providing
internal compression as an option at some future date.

The compression you achieve will depend greatly on the data you are storing.
This means that you won't have an easy way of knowing ahead of time how
much data you can safely put onto a single tape.  In my mind this is a
pretty unpleasant situation to be in.  Fortuantely for us, the 5 Gby
capacity of the EXB8500 drives seems adequate for our needs.

				Ray