z/OS initialization, or an Initial Program Load (IPL), is the act of loading a copy of the operating system from disk into the processor’s real storage and executing it.
This process essentially consists of:
– System and storage initialization, including the creation of system component address spaces
– Master scheduler initialization and subsystem initialization
z/OS systems are designed to run continuously with many months between reloads, allowing important production workloads to be continuously available. Changes are the usual reason for a reload, and the level of changes on a system dictates the reload schedule.
Bear in mind that:
– A test system may be IPLed daily or even more often.
– A high-availability system (from a bank for example), may only be reloaded once a year, or even less frequently, to refresh the software levels.
– Outside influences may often be the cause of IPLs, such as the need to test and maintain the power systems in the machine room.
– Sometimes faulty software uses up system resources that can only be replenished by an IPL. This sort of incident is normally the subject of investigation and correction so that it does not happen again.
Shutting down the system usually requires a single command, which results in the removal of most tasks except for the automation task itself. The automation task is closed manually, followed by any commands needed to remove the system from a Sysplex or serialization ring.