Recommended space

The size specified for the coupling facility list structure in the CFRM policy should be large enough to allow for the recommended number of list headers and list entries, as described below. Specifying more than this minimum amount of space allows MVS™ to queue more signals on a list simultaneously and improves signaling performance.

It is recommended that the coupling facility list structure be large enough to have at least one signal queued simultaneously on every list header used as a signaling path.
The number of data elements needed for these list entries depends on the size of the signals that are sent. For example, for a 61K byte signal (the largest signal supported by the MVS signaling service), 16 data elements would be needed. Since the entry to element ratio is 1:1, 16 list entries would also be allocated. Thus, to allow for one 61K byte signal to be queued simultaneously over every signaling path, the list structure must be large enough to accommodate 16 list entries per path.
It is also desirable for the list structure to be large enough to accommodate the peak number of signals simultaneously queued over each list header for a signaling path.
Otherwise, MVS is forced to queue the signal on the sending system. This delays signal delivery and increases the amount of real storage needed for message buffers.
MVS also needs list entries to manage the coupling facility list structure. The number and size of these list entries varies with:
- The number of list headers in the list structure,
- The maximum number of systems that can connect to the list structure,
- The release of MVS that those systems are running.

In OS/390®, it is recommended that the list structure be large enough to provide one list entry per connected system, plus eight additional list entries for control data. Each list entry requires only one data element.

If fewer than the recommended number of list entries are available, MVS might continue to use the coupling facility list structure for signaling, although possibly with some degradation.