Logger and coupling facility duplexing combinations

In addition to the types of duplexing methods that Logger provides, coupling facility structures can also be duplexed automatically. System-managed duplexing rebuild provides the capabilities of keeping two instances of a coupling facility structure, one a duplicate of the other. When there are two duplicate structure instances, the structure is considered to be in duplex-mode. If there is only one structure instance, the structure is considered to be in simplex-mode.

The Logger log stream LOGGERDUPLEX parameter and your parallel sysplex configuration will determine whether Logger will duplex the log data or whether system-managed duplexing rebuild will occur using duplex-mode coupling facility structures.

The following factors determine the type and whether Logger duplexing will occur with respect to the coupling facility structure environments:

The coupling facility's volatility state
The simplex-mode or duplex-mode of the structure
The connection relationship between the structure instances
Their composite connection relationship with the connecting system
The Logger log stream duplexing options (new keyword LOGGERDUPLEX and existing keywords STG_DUPLEX and DUPLEXMODE).

Table 1 illustrates the type of log data duplexing that will be in effect based on the combination of these factors.

Table 1. Logger and System-Managed Duplexing Rebuild Combinations
Structure Mode / Case #	Logger Duplex	STG_DUPLEX (NO)	STG_DUPLEX (YES)
Structure Mode / Case #	Logger Duplex	STG_DUPLEX (NO)	DUPLEXMODE (COND)	DUPLEXMODE (UNCOND)	DUPLEXMODE (DRXRC)
SIMPLEX
Case 1	UNCOND / COND	Local Buffers	Staging Data Set	Staging Data Set	Local Buffers, Staging DRXRC
Case 2	UNCOND / COND	Local Buffers	Local Buffers	Staging Data Set	Local Buffers, Staging DRXRC
DUPLEX
Case 3	UNCOND / COND	Structure, Local Buffers	Structure, Staging Data Set	Structure, Staging Data Set	Structure, Local Buffers, Staging DRXRC
Case 4	UNCOND / COND	Structure, Local Buffers	Structure, Local Buffers	Structure, Staging Data Set	Structure, Local Buffers, Staging DRXRC
Case 5a	UNCOND	Structure, Local Buffers	Structure, Local Buffers	Structure, Staging Data Set	Structure, Local Buffers, Staging DRXRC
Case 5b	COND	Structure	Structure	Structure	Structure, Staging DRXRC

Note:

In Table 1, the Mode / Case # column is defined as follows:
simplex-mode
Only one structure instance (system-managed duplexing is not in effect):
- Case 1 — a failure dependent connection between the connecting system and the structure, or the structure coupling facility state is volatile.
- Case 2 — a failure independent connection between the connecting system and the structure, and the structure coupling facility state is non-volatile.
duplex-mode
There are two structure instances (system-managed duplexing is in effect):
- Case 3 — a failure dependent connection between the connecting system and the composite structure view, or the structure coupling facility state is volatile.
- Case 4 — a failure independent connection between the connecting system and the composite structure view, and a failure dependent relationship between the two structure instances, and the structure coupling facility state is non-volatile.
- Case 5 — a failure independent connection between the connecting system and the composite structure view, and a failure independent relationship between the two structure instances, and the structure coupling facility state is non-volatile.
The characterizations in Table 1 indicate what type of duplexing would be in effect for the particular case and parameter specifications:
local buffers

Logger is duplexing data in data spaces associated with the IXGLOGR address space.

staging data set

Logger is duplexing the log data in a staging data set associated with the system's connection to the log stream and structure.

Staging DRXRC

Logger is duplexing the log data in a staging data set associated with the system's connection to the log stream and structure, but only for specific disaster recovery purposes. For example, when system IPLed with DRMODE=YES system parameter option and Y reply to message IXG068D. Refers to DRXRC-type staging data sets.

structure

The log stream data is duplexed in two instances of the structure. Logger is not duplexing the log data because the configuration and duplexing options indicate it is not necessary.

The new environments that can exist when the coupling facility structure is in duplex-mode offer additional log data duplexing and recovery opportunities.

Case 3 demonstrates that even though the structure is in duplex-mode the composite view of the structures to the connecting system is failure-dependent (or in a volatile state). There is no recoverability advantage with the two structure instances since there is still a single point of failure dependency. This environment is treated in a similar manner as in the simplex-mode Case 1 above.
Case 4 demonstrates that even though the composite view of the structures to the connecting system is failure-independent (and non-volatile), the two structure instances are failure-dependent so Logger would continue to provide its own duplexing. Since a single point of failure exists for the two structure instances, Logger will continue to use its own duplexing method for log data recovery. This environment is treated in a similar manner as in the simplex-mode Case 2 above.
Case 5 demonstrates that given the appropriate failure isolated relationship between the connecting system as well as between the two structure instances, the duplexing and recovery of the log data can be handled using either the duplex-mode structure or from Logger's duplexed copy.

See System logger recovery for additional details on how Logger handles recovery of log data for the different failure conditions, along with the system-managed duplexing rebuild and single point of failure transitions based on the types of log data duplexing.