The STG_DUPLEX and DUPLEXMODE parameters are as follows:

• STG_DUPLEX(NO), which is the default, specifies that system logger is to maintain a copy of coupling facility data in local storage buffers on each system.

  Choose this option when your application is not vulnerable to data loss that might occur if your environment contains a single point of failure.

  Note that in certain rebuild error cases, system logger will automatically allocate a staging data set, even when you specify STG_DUPLEX(NO). See Plan space for staging data sets for more information.

• STG_DUPLEX(YES) specifies that you want system logger to maintain a copy of coupling facility data in DASD staging data sets, one per log stream per system. See Plan staging data sets for coupling facility log streams for more planning for staging data set duplexing.

  You can only specify DUPLEXMODE with STG_DUPLEX(YES). Choose one of the following:

  • DUPLEXMODE(COND), which is the default, specifies that system logger is to conditionally duplex coupling facility log data for a system ONLY when a system's connection to the log stream contains a single point of failure and is therefore vulnerable to loss of data.

    With DUPLEXMODE(COND), note that different systems connected to a log stream might be backing up data differently. Duplexing is done on a connection basis, depending on whether a system's connection contains a single point of failure.

    If you have specified STG_DUPLEX(YES) DUPLEXMODE(COND), system logger can switch duplexing methods when the system logger configuration changes. For example, if you specify conditional duplexing for a log stream that writes to a coupling facility that changes from volatile to non-volatile, system logger will stop duplexing the data to staging data sets and duplex the coupling facility data to local storage buffers, because there is no longer a single point of failure in the volatile coupling facility.

    If the coupling facility changes from non-volatile to volatile, the coupling facility structure is rebuilt. When the rebuild completes, system logger re-evaluates each connection for a single point of failure and whether to use staging data sets.

    Use STG_DUPLEX(YES) DUPLEXMODE=(COND) when you cannot afford any data loss. It is a flexible way to protect your application against a single point of failure on a connection by connection basis. With DUPLEXMODE=COND, duplexing to staging data sets is only used when needed for a particular connection. Connections that do not have a single point of failure can exploit the performance benefits of backing up log data to local storage buffers when duplexing is not necessary.

  • DUPLEXMODE(UNCOND) specifies that system logger is to unconditionally duplex coupling facility data for log stream connectors to DASD staging data sets. Use this option when you always want to use staging data sets, regardless of whether the connection contains a single point of failure or not. The only possible exception to logger unconditionally using staging data sets for this specification is when LOGGERDUPLEX(COND) is also specified for the log stream. For this case, if the structure is undergoing a system-managed duplexing rebuild into two failure independent structure instances, and if there is no other single point of failure between logger and the structure, logger will not provide any log data duplexing.
  • DUPLEXMODE(DRXRC) specifies that system logger is to unconditionally duplex coupling facility data for log stream connectors to DASD staging data sets, but only for specific disaster recovery purposes. Use this option when you always want to use staging data sets for specific disaster recovery situations, but not use them for any local system log data recovery.

    The type of log data duplexing that occurs for any data written to the coupling facility structure will be as follows:

    • If the structure is in simplex-mode:
      1. duplexing to local buffers for any local sysplex system log data rebuild/recovery use, plus
      2. duplexing to DRXRC-type staging data sets for any remote (hot) system disaster recovery use.
    • If the structure is in duplex-mode:
      1. duplexing to the 2nd structure instance (provided by XES), and
      2. duplexing to local buffers for any local sysplex system log data rebuild/recovery use, when LOGGERDUPLEX(UNCOND) is also specified or there is a failure-dependence configuration, plus
      3. duplexing to DRXRC-type staging data sets for any remote (hot) system disaster recovery use.

    With the DUPLEXMODE(DRXRC) specification, Logger will duplex the log stream data written to a Coupling Facility structure in a staging data set in an asynchronous manner. The DASD mirroring protocol of the staging data set will be done using extended Remote Copy (XRC) when the staging data set volume is part of the XRC DASD consistency group.

    Any log stream staging data set defined for this use will only be usable for recovery purposes when a system is IPLed with the DRMODE=YES specification. This type of IPL would normally be done when a secondary site recovery system is IPLed to handle the start up following a disaster situation for the primary (main) sysplex systems. Once the log stream is recovered in this situation, the log stream attributes are also updated to indicate STG_DUPLEX=NO so no type of staging data set duplexing will be in effect for the log stream. See Plan DRXRC-type staging data sets for coupling facility log streams for more details on establishing the appropriate environment for this duplexing approach.