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.