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From: dhb@rayssd.UUCP
Newsgroups: net.periphs
Subject: Re: Magtape capacities vs densities ?
Message-ID: <344@rayssd.UUCP>
Date: Fri, 13-Jan-84 12:54:49 EST
Article-I.D.: rayssd.344
Posted: Fri Jan 13 12:54:49 1984
Date-Received: Sun, 15-Jan-84 00:27:18 EST
References: <190@rna.UUCP>
Organization: Raytheon Co., Portsmouth RI
Lines: 22

The amount of data that can be stored on a magtape is not strictly
a function of the density, the record size plays a very important
part.  The amount of tape used by each record is approximately the
size of the record (in bytes) divided by the density.  The reason
I say approximately is that there are other things recorded on the
tape than just the data (CRC data and junk like that).  Also, the
number "6250" is just an approximation due to all the crap that
gets put on the tape by the "group-coded-recording" method.  The
real physical density of the tape is 9042 BPI.  Once you have
computed the approximate amount of tape used by the data records,
you have to remember that between each record is an inter-record
gap which is somewhere between .6 inches and .75 inches (depending
on the recording density).  Thus if you write a tape with many
small records, the tape ends up being mostly empty space.  Also,
if you put more than one file on the tape, a file mark takes up
about 3.5 inches.
-- 
	Dave Brierley
	Raytheon Co.; Portsmouth RI; (401)-847-8000 x4073
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