Specifying attributes to handle space constraints during allocation

You can specify attributes on Page 2 of the Data Class Define panel to indicate whether or not to retry new data set allocations or extends on new volumes that fail due to space constraints.

During allocation, there might not be enough space on a volume to meet the requested space. SMS volume selection can sometimes solve this problem by trying all candidate volumes before failing the allocation. You can also use the Space Constraint Relief and Reduce Space Up To (%) attributes to request that an allocation be retried if it fails due to space constraints. SMS retries the allocation by combining any of the following:

Spreading the requested quantity over multiple volumes
Allocating a percentage of the requested quantity
Using more than 5 extents

Space Constraint Relief

Specifies whether or not to retry an allocation that was unsuccessful due to space constraints on the volume. Before it is retried, the allocation is attempted on all candidate volumes. Space Constraint Relief applies only to system-managed data sets, and is limited to new data set allocations, and while extending the data set on new volumes. Specify one of the following:

Y: Specifies that SMS retry the allocation.
N: Specifies that SMS does not retry the allocation, so that allocation is not attempted on multiple volumes.
This is the default.

If you specify Y, SMS begins the retry process. This is a one- or two-step process, depending on the volume count you specified. For JCL allocations, SMS determines the volume count by taking the maximum of the unit, volume, or volser count. If these are not specified, SMS picks up a volume count from the data class. If there is no data class, SMS defaults the volume count to 1.

If the volume count is 1 (one-step process)
SMS retries the allocation after reducing the requested space quantity based on the Reduce Space Up To attribute. SMS simultaneously removes the 5-extent limit, so that SMS can use as many extents as the data set type allows.
If the volume count is greater than 1 (two-step process)
First, SMS uses a best-fit volume selection method to spread the primary quantity over more than one volume (up to the volume count). If this fails, SMS continues with the best fit method after reducing the primary quantity and removing the 5-extent limit.

Tip: You can remove the 5-extent limit without reducing the primary quantity by specifying 0 for the Reduce Space Up To (%) attribute.

For extends to new volumes, space constraint relief is strictly a one-step process. If regular volume selection has failed to allocate space, SMS reduces space or removes the 5-extent limit, but does not try the best-fit method.

The maximum number of extents per volume and the maximum number of volumes per data set vary depending on data set type as follows:

A basic-format or large-format sequential data set and a direct data set can have up to 16 extents per volume and up to 59 volumes.
An extended-format sequential data set can have up to 123 extents per volume and up to 59 volumes. Either all or none of these volumes can be arranged into stripes for parallel processing.
A non-system-managed VSAM data set can have up to 255 extents per component and up to 59 volumes.
A system-managed VSAM data set can have up to 255 extents per stripe and up to 59 volumes. This extent limit can be removed if the associated data class has extent constraint removal specified. Up to 16 volumes at a time can be read or written in parallel due to striping.
A PDS can have up to 16 extents and only one volume.
A PDSE can have up to 123 extents and only one volume.
An HFS data set can have up to 123 extents per volume and up to 59 volumes.

Reduce Space Up to (%)

Specifies the amount by which you want to reduce the requested space quantity when the allocation is retried. You must specify Y for the Space Constraint Relief attribute. Valid values are 0 to 99.

If you request space constraint relief but do not specify a percentage value (either 0 or blank), SMS does not reduce the requested space quantity. This implies your application cannot tolerate a reduction in the space to be allocated, so only the 5 extent limit is relieved.

When you request space constraint relief in one or more data classes, you might notice any of the following:

Very large allocations might succeed if a sufficiently large volume count is specified in the data class or through JCL.
Existing data sets might end up with less space than initially requested on extents.
The space allocated for new data sets might be less than requested.
The number of extents used during initial allocation might result in fewer extents being subsequently available. For example, if the primary space allocation uses 10 extents when allocating a physical sequential data set, then only 6 extents are left for allocation of the secondary quantity.
X37 abends should occur less frequently.