Specifying the coupling facility requirements
When setting up the CFRM policy, the installation also must be
aware of certain coupling facility attributes. If your structure
has the following coupling facility characteristics, you must document
them:
- Nonvolatility — the requirement that a coupling facility must be able to preserve the structure storage over a utility power failure.
- Failure-independence — the requirement that a coupling facility be in a separate configuration from the system accessing it, thus eliminating a single point of failure.
- Coupling facility level — the requirement that a coupling facility have a certain level of coupling facility control code (CFCC).
Additionally, the application must document whether it supports system-managed protocols. If so, the installation must provide at least two coupling facilities at CFLEVEL=8 or higher to support system-managed rebuild. For system-managed duplexing rebuild, at least two coupling facilities at CFLEVEL=11 or higher must be provided. For system-managed asynchronous duplexing, at least two coupling facilities at CFLEVEL=21 or higher must be provided.
Knowing these requirements enables the installation to correctly
specify a preference list of coupling facilities in which your structure
can reside. The system uses the preference list and the SFM system
weights when attempting to allocate the structure. The system chooses
the first coupling facility in the preference list that meets the
following requirements:
- Has connectivity to the system that is trying to allocate the structure (depending on the application's connectivity specifications)
- Has a CFLEVEL equal to or greater than the requested CFLEVEL or with a CFLEVEL that supports system-managed processes if the application specified ALLOWAUTO=YES. Note that a CFRM policy DUPLEX specification of ASYNC or ASYNCONLY will cause a requested CFLEVEL of at least CFLEVEL=21.
- Has available space greater than or equal to the requested structure size
- Meets the volatility requirement requested
- Meets the failure-independent requirement requested
- Does not contain a structure in the exclusion list.
If no coupling facility in the preference list meets all these
requirements, then the system uses the following priorities to attempt
to allocate the structure:
- Without the exclusion list requirement
- Without the failure-independent requirement
- Without the volatility requirement
- In a coupling facility that meets or exceeds the CFLEVEL requirement and has the most available space.