OS/390 System Managed Storage - Data Class

The Dataclass construct defines what a file looks like. The Dataclass ACS routine is always invoked, even if a file is not SMS managed. The Dataclass is one of those constructs that can be used very powerfully, or it can be almost ignored.
A file is described by its

• Dataset organisation
• Record format
• Record length
• Space allocation
• How many volumes it can span over

and that's just flat files. VSAM files have lots of other attributes. If you define a Dataclass for every combinations of these attributes, the list would become difficult to manage. The record length, is a good example, you probably have hundreds of different record lengths in your organisation, and you don't want to have to add a dataclass every time someone wants a new one.
A suggestion is to define a dataclass for each common combination of dataset organisation and record format, and let your users specify record length and space. So the combinations you would specify are

DSORG / RECFM ->	FB	FBA	FBS	VB	VBS
PS	x	x	x	x	x
PO	x	x	x	x	x
PDSE	x	x	x	x	x

Another use of the dataclass is to force a file to go multi-volume. When setting a dataclass up in ISMF, you can specify a number between 1 and 59 in the 'volume count' field. By default, a VSAM file will look for its primary allocation, when it extends to a new volume. If you want it to use the secondary allocation, specify 'SECONDARY' in the 'Additional Volume Amount' Field.

Its probably not worth setting up a dataclass for every variant of VSAM files, but it is a good idea to define some multi-volume dataclasses for VSAM.

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