🚀 go-pugleaf

Michael Gersten writes
> 
> Questions on tape drives.
> 
> A long time ago there was something called a "Stringy Floppy". What this
> was was a tape drive where you could write to block #x on the tape, and
> not destroy block #x+1. This allowed it to be used as a slow floppy
> drive (hence the name).
> 
> Is there anything like this in modern tape drives, or are they strictly
> a "Re-write from beginning to end, only" sequential devices?
> 
> What prices should I expect to pay for a tape drive?
> 

The helitical scan drives (Exabyte and DAT) have problems positioning the tape  
to exactly the same place every time.  On the Exabytes it works like this:
There is an erase head ahead of the write head.  It's about 11 tracks wide.
When ever the drive stops writing for any reason (Slow access, computer is busy  
with something else, whatever) the drive writes 11 "gap tracks"  These are 0  
filled data blocks, with special header info marking them as gap tracks.  When  
the buffer is full enough to start writing again, the drive reverses for 11  
tracks, brings the drum and tape up to speed, reads the last track it wrote,  
reads the gap track, and starts writing after the gap track.

Net effect:
	Writing track x destroys tracks x+1 to x+11.  Writing track x makes it  
impossible to write track x+1.

You do NOT have to start at the beginning however.  I can start writing on a  
tape at any filemark.  You work it like this:
(Exabyte receipe:  Haven't tried with a DAT yet.)
move to the filemark. bsf to the front side of the tape mark.
Write 2 tape marks,
Bsf over one.  Now you can write to the tape.  Anything beyond this is history,  
due to a deficiency in Nexts tape driver.  (The tape drive itself permits  
advancing beyond the double filemark that is the usual signal for end of  
recorded media.  Next's mt command won't let you do it.)

The mechanisms of the tape drive support what you say, and it would be workable  
if you defined blocks as being a large number of tracks.  (On an Exabyte, a  
track = 8 k)  You would ignore the 10 track gaps as being part of your "floppy  
format" headers.  

Note also.  The standard exabyte 8200 has a forward search capacity that is  
only 10 times the reading speed.  This means that it can take 5-10 minutes just  
to position the tape.

The DAT's are  much faster at positioning, averaging about a minute.

Prices:  DATs start at 1500 and go up to about 3000.