The model DSCB for GDGs was not one of IBM's better ideas. The concept
was to insure that all generations of the GDG had the same DCB
characteristics, so you defined the model DSCB with the RECFM, LRECL,
etc. This also facilitated concatinating generations together or doing
a "gdg all" request with no generation number.
In early OS/360 systems where the number of GDGs was probably small,
this may have been manageable but it rapidly became unmanagable as
system sizes and the number of GDGs in an installation grew. So most
shops switched to creating a generic cataloged "model DSCB" with NO DCB
characteristics and using DCB= to refer to it. Keeping the DCB
characteristics of each generation compatible became a programmer
responsibility. Omitting the DCB=model when setting up a new GDG is a
common error.
When SMS is active, the SMS data class is used to specify the DCB
characteristics, even for non-SMS datasets.
--
Bruce Black
Senior Software Developer
Innovation Data Processing
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