What's magtape good for anyway?

[Tim Shoppa]

> > Mag tape has
> > several things that make it difficult, one is old (late 60s and through
> 
> In old movies, filmmakers often focused on spinning tape 
> drives when they wanted to show a computer "thinking."  What is it 
> about tape drives that made them such a powerful symbol for big, 
> complicated computer systems?

You have to realize that disk storage on mainframe systems in the
1960's was usually quite small.  Almost all "large-scale" processing
was from tape drive(s) to tape drive(s).  If you find a really good
reference on sorting and collating (Knuth, for example) a lot of
effort is made on doing things with as little core and disk space
as possible.  Most of these methods are still used today on really
large data sets (for example, FFT's on multi-gigabyte data sets
which are never entirely in memory.)

> > the 70s) drives had a difficult time starting and stopping without 
> > breaking tape or resorting to complex(then standards) controllers.  This 
> > lead to things like large interrecord gaps (start, speed up read, stop,
> > backspace records, stop, read) due to the inerta of starting and stoping 
> > the reels.  Also fixed record sizes were used to make blocks about the 
> > same length so blocks and marks could be differentiated using simple 
> > timers.
> 
> Was dectape an attempt to remedy some of these problems?  My 
> hazy recollection was that you could treat dectape in some ways as if 
> it were a disk.

DECtape was very much different from other tape media of the time.
You didn't treat it as a disk in just some ways, you treated it as
a disk in all ways.

At the time of DECtape, the most inexpensive removable disk media was
the RK05 DECpack, which cost about $150-$200 per platter.  DECtape was
created as a more affordable "disk-like" removable media so that
each user could carry his files around with him.

> > Magtape was for the longest time the only portable media, which lead to 
> > the ansi/EBCDIC problems (Evryone else and IBM/HP).  It was generally 
> > used for archival storage making file organized access excess overhead.  
> > While often used as block oriented, many systems used it more as a stream 
> > device where the high volume storage (relative to the disks of the time) 
> > capability was available.
> 
> How much data can magtape hold?

A 1600 bpi 2400 foot 9-track holds about 40 Megabytes if you use long
blocks.  Other more recent magtapes (i.e. DLT's) hold 40-100 Gigabytes per
reel/cartridge.  Some specialized optical tape media hold Terabytes
per reel.

>  If magtape was a portable media, 
> does that mean that the manufacturers agreed on the width of 
> the tape, the density of recording, the method of recording bits, 
> etc.?

Absolutely.  There are ANSI standards for all of the above.  Despite
what others claim, interchangability was always rather straightforward,
and the worst problems are the "concepts" not supported by some operating
systems (i.e. Unix lacks file support for anything other than a file that's
just a stream-of-bytes).

> I have an old 9 track tape from a computer course I took in 1980.  
> For sentimental reasons I'd love to get a copy of its contents.  Is 
> this possible do you think?

Absolutely.  Part of my current profession is reading 9- (and 7-) tracks
that are up to 35 years old.

> > When processing was done on early system usually two or three drives were 
> > involved as one of two were for reading  and the third was writing results
> > usually due to memory size limitations of the time compared to the amount 
> > of data.  Alot of magtapes lore is a result of historical use.

These uses aren't just historical - many of us still deal with datasets
that are Terabytes in size and which cannot be disk (or core) resident.

Tim. (shoppa at triumf.ca)

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