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From: cmf851@anu.oz.au (Albert Langer)
Newsgroups: comp.periphs.scsi
Subject: Random Access DAT
Message-ID: <1991Jun14.211246.4340@newshost.anu.edu.au>
Date: 14 Jun 91 21:12:46 GMT
Sender: news@newshost.anu.edu.au
Organization: Computer Services Centre, Australian National University, Canberra, Australia.
Lines: 95


1. Now that DAT is available with fairly high speed random access, is
there any software available to actually USE this? (Or for Exabyte
which is only slightly slower, or indeed any older software to use
seeks on a raw character device?)

2. Do the drivers normally provided on Unix systems (e.g. ISC 386/ix 2.2)
allow for random seeks or would new drivers as well as application
software be needed? (If so, are any such drivers available?).

3. Main application I have in mind is for offline archiving of large
volumes of files to be available for mail requests. Tape could be
written once, or with supplements added in batches after filemarks.
Requests get batched or handled individually but either way 
spin the tape at high speed between locations where actual file
data has to be extracted rather than scanning the data at ordinary
speed (and tying up the CPU with scanning).

4. Is it practical to do this with filemarks instead of fully
random seeking? (i.e. THOUSANDS of filemarks?) Seems undesirable
to me as there would be a stop-start after each logical file was
written, and also I imagine that scanning for a filemark is slower
than scanning to a specified block count? Is that right?

5. A nice way to do it might be something that makes a directory
showing the byte offset to each file as files are being streamed
out through cpio or tar. Then to read you just sort the requests.
Normal recovery through cpio or tar would be available,
and faster direct access as well.

6. I understand that dataDAT allows random writing as well, but only
DDS is popular which does not allow random writing. Is there any way with DDS
to leave huge gaps and waste tape in order to allow a primitive form
of random writing between file marks or pseudo end of media marks?
Seems to me that should be PHYSICALLY possible regardless of the
format, but I can't figure out how to do it. Alternatively, or perhaps
it is the same question, is there some way to use numerous "partitions"
with DDS instead of just two?

7. If such software isn't yet available how about some DAT vendors
(or even SCSI adaptor vendors) developing some? Seems to me it would
widen the DAT (and/or Exabyte) market beyond backup streaming
(and incidentally provide for fast recovery of files from backups).
Once a crude prototype was freely available from a DAT vendor, others
would develop new and better applications.
Would also widen appeal of SCSI (less substantially).

8. If this isn't the best newsgroup to ask in, which should I try?

P.S. While I am at it, an entirely separate issue, which also
happens to relate to Random Access DAT.

I have read of VERY expensive "juke boxes" for tapes as well as for
WORMs and CD ROM.

9. Is there any reason a crude version of such a juke box could not
be constructed quite cheaply? (e.g. a rather slow one, using toy
components?) It doesn't sound all that complex to me for a toy robot
to pick tapes from a rack and drop them into a slot and vice versa.
Anybody working on this?

10. Are audio juke boxes for DAT available or planned? If so is
anybody going to convert them for computer use? (Just take the
the electronics from a computer DAT and use the mechanical parts
from an audio juke box).

Hmm, that reminds me of another issue.

11. The Wangtek DAT I am using has a JVC drive and an entirely
separate Wangtek SCSI controller which is merely screwed together
to make it look like a single full height drive. Would the JVC
drive be an ordinary audio DAT drive and would there be some
simple way one could feed output from an ADC to it or input to
a DAC from it (at least standalone audio DACs are readily
available for use with high end CD players)?

12. Likewise, could one just add a SCSI controller board to
a consumer audio DAT drive to get a computer DAT (as I
believe Wangtek may have done)? I guess consumer audio DAT
will soon be dramatically cheaper than before and it
might be worth looking into. (Or are computer DAT drives
expected to quickly follow consumer audio prices down?)
Seems to me that with CD ROMS there may be some technical
differences in seek-time requirements etc to explain a
continuing large price differential, but for DAT the
ONLY technical differences should be in controller
electronics and price differentials may be purely a
marketing decision that could be challenged by some
small PCB producer buying audio drives and adding their
PCBs.


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