Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!utgpu!water!watmath!clyde!rutgers!ames!cit-vax!mangler
From: mangler@cit-vax.UUCP
Newsgroups: comp.sys.misc,comp.periphs,comp.unix.wizards
Subject: Re: magtape, and what's hot
Message-ID: <2714@cit-vax.Caltech.Edu>
Date: Sat, 16-May-87 23:46:33 EDT
Article-I.D.: cit-vax.2714
Posted: Sat May 16 23:46:33 1987
Date-Received: Sun, 17-May-87 08:48:47 EDT
References: <878@wjvax.wjvax.UUCP> <242@uwslh.UUCP> <6567@mimsy.UUCP>
Distribution: na
Organization: California Institute of Technology
Lines: 27
Xref: utgpu comp.sys.misc:566 comp.periphs:354 comp.unix.wizards:2157
Summary: interrecord gaps count double

In article <6567@mimsy.UUCP>, chris@mimsy.UUCP (Chris Torek)
calculates the average throughput of 6250-bpi tape, and gets
the right answer despite two misconceptions.

The ANSI standard gap length for 6250-bpi tape is 0.3 inches,
Most manufacturers fudge on this, and come out somewhere around
0.4 inches; certainly well under the 0.7 inches that Chris assumed.

During the interrecord gap, the tape is accelerating/decelerating,
and on average will be moving at half normal speed.  So a gap of
0.4 inches will take as much time as 0.8 inches of normal motion.

		     speed * blocksize
Throughput  =	---------------------------
		 2*gap + blocksize/density

For 125 ips, 0.4 inch gap, 6250 bpi, 32KB blocks  =  678 KB/s.
For 125 ips, 0.7 inch gap, 1600 bpi, 10KB blocks  =  164 KB/s.

The 1600-bpi figure is consistent with the peak speed I measure
on my TU77, doing write() in a tight loop.  Dump throughput is
only 80-90% of this (we have tens of thousands of tiny files).

Despite the details, Chris's conclusion still stands.  (I'd say
the point of diminishing returns is about 45 ips @ 6250 bpi).

Don Speck   speck@vlsi.caltech.edu  {seismo,rutgers,ames}!cit-vax!speck