Automatic altering of structure size
z/OS® can automatically expand or contract the size of a VSO structure in the coupling facility if it needs storage space.
Enabling this function for preloaded VSO DEDBs can prevent wasted space; however, you must be careful with this function when VSO DEDBs are loaded on demand.
Recommendation: If you preload your shared VSO DEDB areas, do not use the automatic alter function. The automatic alter function might reclaim any unchanged data in the cache structure.
To ensure correct sizing and that the automatic alter function is disabled for your preloaded shared VSO DEDB areas, specify the following parameters in the area's CFRM policy:
- INITSIZE(x)
- Specifies the initial amount of space to be allocated for the structure in the coupling facility. You can estimate this size using the IBM® System z® Coupling Facility Structure Sizer Tool (CFSizer). To learn more, see CFSizer.
- SIZE(y)
- Specifies the maximum amount of space to be allocated for the structure in the coupling facility.
- ALLOWAUTOALT(NO)
- Specifies whether to allow system-initiated alters (automatic alter) for this structure. For preloaded shared VSO DEDB areas, you should specify ALLOWAUTOALT(NO).
If the size specified by INITSIZE is too small, IMS will alter the size of the structure to the needed value as calculated by IXLCSP, up to the size specified in the SIZE parameter.
Related Reading: For information about the CFRM parameters that enable automatic altering of structures, see z/OS MVS Setting Up a Sysplex