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 volumes. Allocations that might have failed for lack of space can succeed.

The attributes are:

Space Constraint Relief (values are YES or NO). This specifies whether or not to retry an allocation that was unsuccessful due to space constraints on the volume.
Reduce Space Up To % (0 - 99%). Used with Space Constraint Relief, this 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 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 volume. During EOV processing, space constraint relief affects the primary or secondary allocation amount for VSAM data sets, or the secondary allocation amount for non-VSAM data sets. During CREATE processing, the primary quantity might be reduced for both non-VSAM and VSAM data sets.

Exception: The system does not use space constraint relief when data sets are extended on the same volume.

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

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.
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.

Recommendation: 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.

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