Re: File-system on tape
Kenneth.Scharf@coulter.com wrote:
> > On Tue, May 26, 1998 at 10:15:58PM -0600, Lazar Fleysher wrote:
> >> Is it possible to create a file system on a tape drive ( like on
> >> mainframes) and use it as a disk? I know it is very slow, but is it
> >> possible?
> >
> > That is really very sick and twisted to even think of such a thing...
> > hmm I wonder why I never thought of it :)
>
> QIC tapes (FTAPE) use the floppy disk controller and are formatted like
> floppy disks with 'tracks' and 'sectors' and so are block replaceable.
> This means that they CAN hold a random access file system just like disks,
> only SLOWER.
Don't be too sure. Most tape drives only allow to write appending to the *end* of
any data already written, so even if random positioning is possible the device is
fairly useless. If you have to rewrite a block on the "filesystem" you would not
be able to go back and rewrite over where it was before without losing any data
after it. Does this mean it would not be possible to write a filesystem which
would exist on a tape? No. It does mean that such a filesystem (especially on a
low-capacity low-bandwith device like QIC over a floppy interface) would be
nearly useless.
I'm speaking here from experience because I have a home-grown backup solution
where I wanted to be able to write a fixed length tape label onto the beginning
of each tape and rewrite it each time I added another file onto the tape. I was
very disappointed to learn that this was not possible.
> QIC tapes have from 20 to 80 parallel tracks on them,
> switching tracks is done by moving the head up and down (just like the old
> 8 track tapes). DEC made a QIC tape drive called DEC-TAPE II that
> interfaced with an rs-232 link. It was random access (just like the
> original dec-tape), each tape holding about as much as an old 8" SSSD
> floppy.
> I remember running the dungeon (aka zork) game on an lsi-11 (pdp-11/03)
> machine with dual dec tape II drives. Yuchk was it slow!
--
Jens B. Jorgensen
jjorgens@bdsinc.com
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