Allocation of data sets with the space constraint relief attributes

To reduce allocation failures, three data class attributes can influence the allocation and extension of data sets to new and current volumes. Allocations that might have failed for lack of space can succeed. For a description of each attribute, their values, and their functions, see the following table:

Attributes Values Function
Space Constraint Relief YES or NO This attribute specifies whether to retry an allocation that was unsuccessful due to space constraints on the volume. Space Constraint Relief is disabled by default and as of V2R2, Space Constraint Relief also applies to secondary space.
Reduce Space Up To % 0 - 99% When used with Space Constraint Relief, this attribute specifies the amount by which you want to reduce the requested space quantity when the allocation is retried.
Dynamic Volume Count 1 - 59 or blank This attribute is used during allocation processing to determine the maximum number of volumes a data set can span. It allows the number of primary volumes to increase, if necessary, without adding any candidate volumes to the catalog.

Allocations and extends to new volumes proceed normally until space cannot be obtained by normal means.

The system performs space constraint relief in two situations: when a new data set is allocated and when a data set is extended to a new or current volume. During CREATE processing, the primary quantity might be reduced for both non-VSAM and VSAM data sets.

Space constraint relief, if requested, occurs in one or two methods, depending on the volume count that you specified for the failing allocation.

  1. If the volume count is greater than 1, SMS attempts to satisfy the allocation by spreading the requested primary allocation over more than one volume, but no more than the volume count specified.
  2. If method 1 also fails or if the volume count is 1, SMS modifies the requested primary space or the secondary space for extension, by the percentage that you specified in the REDUCE SPACE UP TO parameter.
The allocation fails as before if either or both methods 1 and 2 are not successful. Two fields can be used to track the use of space constraint relief for secondary allocation:
  • SMF64SSR: Indicates whether secondary space reduction was performed using the attributes that are specified for Space Constraint Relief.
  • SMF64NTA: Indicates the size of extent that is returned in tracks.
For more information about the secondary space reduction support, see the following documentation:
Tip: You can specify 0% in the data class for this parameter so space is not reduced.

SMS removes the 5-extent-at-a-time limit. (For example, sequential data sets can have a maximum of 16 extents.) Without this change, the system tries to satisfy your primary or secondary space request with no more than five extents. If you request a large amount of space or the space is fragmented, the system might need more than five extents.

Restriction: VSAM and non-VSAM multistriped data sets do not support space constraint relief. However, single-striped VSAM and non-VSAM data sets use space constraint relief.

For more information about the data class attributes, refer to z/OS DFSMSdfp Storage Administration.