Authorized application specification

When requesting an XES service to connect to a structure, the application optionally can specify a size for the structure. The system uses the smaller of the two sizes (as specified in the CFRM policy or by the application), as the target allocation size for the structure.

The application also is required to specify certain structure attributes when connecting to a structure. These structure attributes are used by the coupling facility control code when determining how to most efficiently allocate the various parts of the structure in the coupling facility. Some examples of structure attributes are data element size, whether locks are used, the number of list headers, and whether an adjunct area is required. The coupling facility control code evaluates each attribute in the following sequence:

Available space
The structure is allocated as large as possible based on the available storage in the requested coupling facility. The target size is derived from either the CFRM policy or the application's request to connect to the structure.
Fixed structure controls
Storage for the fixed structure controls used by the coupling facility is allotted first from the total available structure storage. The amount of storage required for the structure controls is dependent on the CFLEVEL of the coupling facility and the maximum structure size defined in the CFRM policy. Therefore, it is important to consider both dependencies when specifying structure size. If too large a portion of the available storage is allotted to the structure controls, there may not be enough storage left for allocation of the remaining structure objects, and the structure in effect will be unallocatable or unusable.
Entry/element ratio
Within the remaining structure storage, once the fixed structure controls have been allocated, the coupling facility control code attempts to allocate entries and elements in a way that most accurately approximates the requested entry/element ratio. If the structure has been allocated with a size less than the minimum structure size required by the specified structure attributes, then the entry/element ratio may deviate from the requested value. If the structure has been allocated with a size greater than or equal to this minimum structure size, then the entry/element ratio should be satisfied.
Entry and element counts
Within the remaining structure storage, once the fixed structure controls have been allocated, the coupling facility control code attempts to maximize the actual number of entries and elements (as specified by the ratio) that can be placed in the structure.

For a description of the structure attributes that an authorized application can specify, see z/OS MVS Programming: Sysplex Services Guide and z/OS MVS Programming: Sysplex Services Reference.