SPACSECB—reduce secondary allocation to best fit (used with StopX37/II)

The SPACSECB function reduces the size of the requested secondary allocation to the size of the largest single extent on the volume (the best fit), even if the request could be satisfied within five extents.

z/OS normally satisfies a secondary extent by allocating up to five individual extents to accumulate the required space. However, this process has two undesirable effects:

• The space on the volume can become more fragmented, because a single large extent might be split to provide the second, third, fourth, and fifth extents of the secondary allocation.
• Five of the maximum extents allowed for a data set (16 extents for non-VSAM data sets) are used instead of one extent, reducing the maximum possible expansion of the data set.

SPACSECB changes the secondary allocation amount before z/OS tries to allocate extents. This process generally provides a net increase in total space allocation.

For more information, see: