3 May
2002
3 May
'02
10:02 a.m.
Re:
No. You haven't been around tape drives long
enough. The tapes
are not preformatted with block headers. They are written sequentially.
In general, a tape drive can only record forward and add data beyond
whatever has already been written.
Quite true! The act of doing: write or read, ..., backspace, and
then write leaves write vulnerable to something called "crap in the gap".
This was explained to me by designers of an HP 9 track drive
back in 1979, when I was re-writing STORE (the backup program
for the Classic HP 3000 on MPE V).
Essentially, I was told that the standards for 7-track and 9-track tape
don't allow any writes after a backspace or a read.
(You can write after a rewind, of course.)
Of course, some OS's and some programs do that (e.g., appending to a tape).
The reason for not allowing such writes had to do with old data/noise
on the tape that might not be erased because backspace and read
operations don't turn on the erase head (obviously), and when you
then do a write, there's a good chance some junk (the "crap") would
be left in the gap between the prior record and the newly written
record ("gap").
Again, IIRC, they said one could probably get away with it, but you
wouldn't be *safe* doing it.
StanStan Sieler sieler(a)allegro.com
www.allegro.com/sieler/wanted/index.html www.allegro.com/sieler