In Figure 1, a log stream has been defined with STG_DUPLEX=YES DUPLEXMODE=COND. There are two systems connected to the log stream. System 1, SYS1, shares the CPC with the coupling facility, so the data that SYS1 writes to the coupling facility is duplexed to a staging data set because there is a single point of failure between the system and the coupling facility. For example, a machine error might require a reset of CPC1, which would result in the loss of both the coupling facility and SYS1 with its local storage buffer copy of the coupling facility log data if it was not duplexed to staging data sets.

System 2, SYS2, is in a different CPC from the coupling facility it writes to, so the backup copy of coupling facility log data for that system connection is maintained in a local storage buffer within SYS2. If SYS2 should fail, its data is still in the coupling facility list structure.

If the coupling facility fails, SYS2 still has the coupling facility log data for this connection in the local storage buffer. The local storage buffer copy of the coupling facility data can be used to recreate the log stream in another coupling facility list structure.

Figure 1. Example: Using Staging Data Sets When the Coupling Facility Shares a CPC with a System

If the installation had specified STG_DUPLEX(YES) DUPLEXMODE(UNCOND), both systems would duplex to staging data sets, even though there is no single point of failure for SYS2.

If the installation had specified STG_DUPLEX(NO), neither system would duplex to staging data sets (under normal circumstances).