Allocating Space for VSAM Data Sets

When you define a data set, you or SMS must specify the amount of space to allocate for the data set. If SMS is active, you can specify a data class and take advantage of the space allocation set by your storage administrator. If you want to specify space explicitly, you can specify it for VSAM data sets in units of records, kilobytes, megabytes, tracks, or cylinders. To maintain device independence, specify records, kilobytes or megabytes. The amount of space you allocate depends on the size of your data set and the index options you selected. Using Index Options explains the index options that improve performance.

If a guaranteed space storage class (STORAGECLASS parameter) is assigned to the data set and volume serial numbers are specified, primary space is allocated on all specified volumes if the following conditions are met. If these conditions are not met, the command fails and IGDxxxxI messages are printed:

All volumes specified belong to the same storage group.
The storage group to which these volumes belong is in the list of storage groups selected by the ACS routines for this allocation.

You can specify space allocation at the cluster or alternate-index level, at the data level only, or at both the data and index levels. It is best to allocate space at the cluster or data levels. VSAM allocates space if:

Allocation is specified at the cluster or alternate index level only, the amount needed for the index is subtracted from the specified amount. The remainder of the specified amount is assigned to data.
Allocation is specified at the data level only, the specified amount is assigned to data. The amount needed for the index is in addition to the specified amount.
Allocation is specified at both the data and index levels, the specified data amount is assigned to data and the specified index amount is assigned to the index.
Secondary allocation is specified at the data level, secondary allocation must be specified at the index level or the cluster level.

VSAM acquires space in increments of control areas. The control area size generally is based on primary and secondary space allocations. See Optimizing Control Area Size for information about optimizing control area size.