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Path: utzoo!linus!philabs!seismo!hao!stcvax!lat
From: lat@stcvax.UUCP (Larry Tepper)
Newsgroups: net.periphs
Subject: Re: Magtape capacities vs densities ?
Message-ID: <218@stcvax.UUCP>
Date: Wed, 4-Jan-84 12:13:32 EST
Article-I.D.: stcvax.218
Posted: Wed Jan  4 12:13:32 1984
Date-Received: Fri, 6-Jan-84 01:21:52 EST
References: <190@rna.UUCP>
Organization: Storage Technology Corp.  Louisville, CO
Lines: 45

Here is a program to compute the capacity you can expect with a given
tape length and density.  Tape records have 82 bytes of preamble and
postamble (total) and are followed by a .6 inch inter-record gap.

The amount of data you can get on a tape depends on the tape-record
size.  If you write 1-byte tape records, you would incur an overhead
of 82 bytes header and .6 inches of gap for every byte.

The 145 Mb number is basically correct -- if you write 16k-byte (not
8k-byte) records and use every inch of the 2400 foot tape.  Note that
a number of popular disks have 32 512-byte sectors (i. e. 16k bytes)
per track

In the program below, the usual value for `data record size' is 512.
A typical value for `blocking factor' is 20 (tar and dump tapes).

#include <stdio.h>

main()
{
	double nrecs, tpsize, recsize, blkfctr, density;

	for ( ;; ) {
		printf("\n Enter tape density (bpi): ");
		scanf("%f", &density);
		printf(" Enter tape length (feet): ");
		scanf("%f", &tpsize);
		if (tpsize > 2400.) {
			printf(" length > 2400???  nice try!\n");
			continue;
		}
		printf(" Assuming use of 95%% of specified tape length\n");
		tpsize *= .95 * 12.;		/* Convert to inches */
		printf(" Enter data record size (bytes): ");
		scanf("%f", &recsize);
		printf(" Enter blocking factor: ");
		scanf("%f", &blkfctr);
		recsize *= blkfctr;
		nrecs = tpsize / (((recsize + 82.) / density) + .6);
		printf(" Number of records per tape:\n");
		printf("\t- data records:  %8.1f\n", nrecs * blkfctr);
		printf("\t- tape records:  %8.1f\n", nrecs);
		printf("\t- megabytes:     %8.1f\n", nrecs * recsize / 1000000.0);
	}
}