Chapter 3. z/OS 87
For a discussion of Failure Independence, please refer to Coupling Facility Configuration
Options, GF22-5042.
3.3.3 Structure rebuild considerations
To be able to do a rebuild of a structure the following criteria has to be met:
򐂰 At least one active connection to the original structure instance
򐂰 Agreement of all the connected instances of the exploiter code on rebuilding the structure
(that is, all of them have connected with rebuild allowed)
򐂰 For the rebuild process to be considered as a viable means of recovery or maintenance:
There must enough physical resources available in terms of coupling facilities defined
in the active preference list and their processor storage to allow the creation of the new
(and temporarily duplicate) instance of the structure.
All the potential and active exploiters of the structure must have connectivity to the new
instance of the structure.
q When repopulating a CF, always use the POPULATECF parameter of the SETXCF
START,REBUILD command. This rebuilds the structures serially, thereby having less
impact on performance. Also, POPULATECF will move XCF structures if they need to be
moved—using SETXCF START,REBUILD with LOC=NORMAL will
not move XCF
structures.
3.3.4 Structure duplexing
Structure duplexing involves maintaining a duplicate copy of a structure in another CF that
may be used in the event of an unplanned outage.
It is specified through the DUPLEX parameter on the STRUCTURE statement in the CFRM
policy, which can have the following values:
򐂰 DISABLED
Neither user-managed nor system-managed structure duplex rebuild can be started for
the specified structure. This is the default.
򐂰 ALLOWED
An application can initiate its own user-managed or system-managed structure duplex
rebuild for the specified structure, or it can be requested via the SETXCF
START,REBUILD,DUPLEX operator command. No attempt is made by the system to restart
duplexing should it be stopped for any reason.
򐂰 ENABLED
The system will initiate a user-managed or system-managed structure duplex rebuild for
the specified structure. In addition, the system will initiate a restart of duplexing should it
be stopped for any reason. To stop structure duplexing requires a CFRM policy change to
one of the other options.
User-managed structure duplexing
User-managed structure duplexing was introduced with OS/390 1.3 with APAR OW28460
and is valid for cache structures only. List and lock structures are not supported. It was
developed to address the shortcomings associated with user-managed structure rebuild
where it is impossible or impractical for the structure exploiters to reconstruct the structure
data when it is lost as a result of failure.

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