CLOCKxx and the sysplex

CLOCKxx also allows you to specify that the system use an external time reference for sysplex operations. In a sysplex, each MVS™ system shares a clock that provides synchronized time stamps. This requirement allows the sysplex to monitor and sequence events across member systems. Systems that run on different processors in a sysplex require a Sysplex Timer to synchronize different TOD time stamps from the processors. Systems that run on a single processor in a sysplex (MVS systems running under VM as guest systems, or systems running in logical partitions in a PR/SM™ environment) can use the TOD clock in the processor to allow the sysplex to control timing events.

Plan the local time for CLOCKxx carefully. To maintain the integrity of time stamps within the sysplex, the standard time origin for the TOD clock must always be the same. Ensure that the TOD clock for each system in the sysplex is set to the same standard time origin. IBM® strongly recommends the use of Greenwich Mean Time (GMT).

Consider those occasions when you want to adjust local time, such as the initiation of Daylight Savings Time in a system or sysplex. If you need to change the time zone, for example, you can change the time without resetting the TOD processor clocks. For a sysplex that uses the Sysplex Timer, you can adjust the time offset from the Sysplex Timer console, or use CLOCKxx and the SET CLOCK command to reflect the new time. For a sysplex that does not use the Sysplex Timer, you can use CLOCKxx and the SET CLOCK command. The changes that you make do not reset the TOD clock in the processor.

When you make adjustments to local time, IBM recommends that you do not reset the TOD clock on a processor in a sysplex. If you reset the TOD clock on a processor in a sysplex, the change affects sysplex timing.