I just went through the messages in the digest on this subject, and I'm
glad to see that collectively we finally got most of it right.

In summary the maximum theoretical capacity is easy to calculate:

6250 (bytes per inch)   times   12    times   (length in feet).

The catch is the inter-record space which is really an inter-BLOCK space
and is esentially lost space.  You maximize the data that you get on the
tape by writing really BIG blocks.   I would suggest blocks around 60,000
bytes, but there may be some restrictions on how high you can go.  This
gives blocks about 10 inches long, so you get to use about 85% of the space
on the tape for data.

I haven't done this since MPE/V.  There were some additional catches that
may or may not apply on MPE/iX.

Reblocking using FCOPY was a very inefficient and time-consuming process.
If you wanted speed and were not concerned with packing the tape, you used
a file equation to redefine the blocksize as the record size (the gory
details were in the FCOPY manual).  The blocks on tape were then the same
size as the blocks on disc and FCOPY thought it was copying records so it
never did any reblocking.  I would hope, however, that the MPE/iX systems
are fast enough that the reblocking won't be a problem - on a Series 30
this was a very major consideration!

If you are writing a big file and it won't all fit on one tape, then you
HAVE to use labeled tapes.  If you need this, note that HP uses the volume
number of the first tape as a volume set number for the whole set of tapes
and this can be quite misleading when the computer asks for the next tape
in the set to be mounted.  ( I'm not sure I'm using the right nomenclature,
but you get the idea.)  I seem to remember that you have to label the tapes
before you start, because there is no mechanism to write the label on
anything except the first tape. I don't know whether this is documented
anywhere - I got it from a "Communicator" quite a few years ago.


John D. Alleyn-Day
Alleyn-Day International
408-286-6421
408-286-6474 (Fax)
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http://www.Alleyn-Day.com