Re: FBA rant was Re: IBM S/360 series operating systems history

Blaicher, Chris wrote:
The limit for CKD volumes is a little more than 54GB. I come up with a
number closer to 500GB. Past that and IBM will need to go to logical
volumes on a physical volume. The reason is the CCHHR count field.

Max CC is FFFF, which give 65536 cylinders (don't forget cylinder 0)
Max HH is E, which gives 15 tracks per cylinder

That gives: 65536 cylinders times 15 tracks times 56664 bytes = 983040 tracks *
56664
983040 tracks * 56664 = 557,053,378,560


Actually it is much larger. You can have more than 15 tracks per
cylinder. I think 3330's had 30. HH can also be FFFF, and R can
be FF. So the max records per volume is FFFF FFFF FF or 16**10.

If you ignore data chaining, the max record size is FFFF (CCW
length field). This make the max size 16**14 bytes or approx.
7 * 10**16 if I did my conversion correctly.

There is also the BB part of the address which is currently
unused (as far as I know).

Any such change also requires a lot of software changes by
IBM, ISVs, and customers.

Other than increasing the number of cylinders, there has not
been a change in MVS disk geometry in quit some time.
I remember having problems going from 2314 to 3330 to 3350.
We found code that hard coded the values for track capacity
and had to be changed. Hopefully most of that kind of code
is gone know, but who knows.

This is probably easier than FBA at this point, but I don't
really know. Any choice will cause some problems, but something
will have to be done eventually.

--
Richard

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