Magnetic tape filesystem

[Jon Elson]

On 02/09/2015 12:28 AM, John Wilson wrote:
> On Mon, Feb 09, 2015 at 10:50:32AM +1000, Tom Sparks wrote:
>> I am wondering if there were any magnetic tape filesystem?
> I have a vague memory of hearing about something (one of the Unices maybe?
> although handling tapes well is WAY out of character for them) that would
> attempt to use 9-track tapes as block-replaceable media.  Which I think
> meant using the "erase" command to add 3" of nothing after each block so
> that you had at least some chance of rewriting that block w/o trashing what
> comes after
Hmmm, that's interesting.  At least on a lot of minicomputer 
tape controls, I THINK
this would be impossible.  There's no way to make it move 
tape with the write gate
on without generating bit clocks, which would write 
transitions on some channel
of the tape.  I'm pretty sure the DEC TM11 and similar units 
couldn't do this.  (I suppose maybe
there was such a mode in the driver/hardware that I'm 
totally unaware of.)

Some other systems may have had specific erase commands or 
options that could
alter the length of the tape gap.  In fact, I think the 
standard interrecord gap
at 800 BPI NRZI WAS 3 inches.  At 1900 BPI PE, the standard 
gap was reduced
to 0.6 inches to improve capacity.  I figured out that I 
could write tapes at
800 BPI on my CP/M system with a gap around 0.5" and other 
systems had
no problem reading them.  On those little 7" reels, you need 
to work to retain
what little capacity there was.  (No replacement of 
individual blocks was ever
attempted on that system.)

Jon