🚀 go-pugleaf

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