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 IGD
xxxxI 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.