As far as I know, your assumption about the naming of the "-27" is
incorrect. The "-3" and "-9" DASD volumes actually existed as physical
3390 formats. An emulated "-27" is 3 times the capacity of a "-9" (just
like the "-9" was 3 times the capacity of a "-3").
In the beginning, the lord IBM created 3390 models 1, 2 and 3, with
1113, 2226, and 3339 cylinders, for reasons known only to the design
engineerings. It just happened that the byte capacities of those disks,
if all tracks were completely written with maximum size full-track
records, was close to 1GB, 2GB, and 3GB. This is probably just a
coincidence.
Then can the model 9, which had 10017 cylinders (3339*3) so it was 3
times the size of a 3390-3, and held about 9GB.
But the -27 is 32760 cylinders, which is close but not exactly 3 times
10017 (10017*3=30051), so the -27 is actually closer to a 27GB disk than
the -9 was a 9GB disk (if my memory is right, without doing the math, it
is actually about 8.5).
the -54 is actually twice the size of the -27 (32760*2=65520) and is
close to a 54GB disk.
Of course, since no one writes full-track records on disk, and the
effective capacity of a track decreases as the blocksizes goes down, the
actual capacity of these disks is somewhat less, depending on the
applications.
--
Bruce A. Black
Senior Software Developer for FDR
Innovation Data Processing 973-890-7300
personal: [EMAIL PROTECTED]
sales info: [EMAIL PROTECTED]
tech support: [EMAIL PROTECTED]
web: www.innovationdp.fdr.com
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