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From: mo@seismo.CSS.GOV (Mike O'Dell)
Newsgroups: comp.periphs
Subject: Fast Tape drives
Message-ID: <43735@beno.seismo.CSS.GOV>
Date: Wed, 6-May-87 15:06:52 EDT
Article-I.D.: beno.43735
Posted: Wed May  6 15:06:52 1987
Date-Received: Sat, 9-May-87 00:50:44 EDT
Organization: Center for Seismic Studies, Arlington, VA
Lines: 46

In a recent posting here Chris Torek posted some
calculations showing that 125 ips 6250 tape drives
were "unnecessary" since you couldn't drive them
full speed.

HORSE HOCKEY!!!

The problem with his calculations is that UNIX
doesn't do overlapped I/O on raw devices, tape or disk!
This means that the REAL TIME for the operation
on the tape drive is EVERYTHING!  While you may
not be able to drive it full speed all the time,
you can come pretty close quite often, and with
real, high-performance start-stop drives, unlike
"streamers", the operations always take just about
the same time, ie, not long.  I have a Fujitsu
streamer on my SUN 3/280, and it streams some
large part of the time doing dumps, but when it
stops, backs up, and has to get a running start
again, it takes forever by comparison. Further,
if you every use the tapedrive for anything besides
dumps, like really writing tapes full of data,
the speed of the drive matters even more since
you probably aren't writing the gargantuan blocks
that dump uses to get its efficiency.

I had a TELEX 8350 (8530??) triple density drive
back on a VAX/780 in '82 and have sorely missed
it since I left that site. 

It's sorta like a V8 engine - turbo-charged
high-compression 4-cylinders can go quit fast
when you get them wound up, but the winding
takes a while.  A V8 goes fast quickly.
(V12's are even better! ;-) )

Nothing beats a REAL tapedrive, i.e.,
1600/6250 bpi @ 125 ips start/stop, ideally
with 200 ips rewind and fast-forward speeds.

Of course these specs are a compromise - until
you see 250ips @ 6250 gcr mode, you ain't seen
a tape drive.

	Yours for faster files, and dumps,
	-Mike le.
le.