What an IPL is and when it happens
An IPL (initial program load) is how a z/OS system starts: the hardware reads a small start-up program from a disk, and that program builds the running operating system. Sites IPL rarely and on purpose, usually to bring in maintenance, so an IPL is planned and approved like any other production change.
Starting a system that is meant never to stop
A laptop boots; a mainframe system IPLs. IPL stands for initial program load. An operator (or automation) tells the hardware which disk to start from, the processor reads a short program from that disk, and that program loads and initialises z/OS. The word is used as a verb too: "we IPL MPSYS on Sunday".
z/OS is built to run for weeks or months between IPLs. Most changes, such as new programs, new datasets, many parameter changes and even some maintenance, can be activated while the system is running. An IPL is needed when something can only be picked up at start-up, or when the system cannot recover any other way.
Why sites IPL
| Reason | Example |
|---|---|
| Planned maintenance | Applying z/OS service (PTFs) that changes modules only loaded at start-up |
| Upgrades | Moving to a new z/OS release or new hardware |
| Configuration changes | A change to start-up parameters that cannot be made dynamically |
| Recovery | The system is in a disabled wait state or cannot be recovered without a restart (an unplanned IPL) |
Where you start it from
Systems run in LPARs (logical partitions) on a machine. Operators manage them from the Hardware Management Console (HMC): activating an LPAR, then performing a Load, which needs a load address (the disk to start from) and a load parameter. Many sites save these values in HMC activation profiles so that the same, checked values are used every time.
Planned like a change
- An approved change record, a time slot agreed with the business, and a run book with exact values.
- A backout: usually the previous system residence volume, kept so that the system can be IPLed again at the old level if the new one has problems.
- In a Parallel Sysplex, systems are IPLed one at a time (a rolling IPL) while the others keep the service running.
- Checks afterwards that prove the system came up with the intended configuration.
What does IPL stand for?
Show a hint
Three words: the first program the hardware loads.
Show the solution
Initial program load.
Common mistakes
An IPL is a production change with approvals, a run book and a backout, even when it is planned every month.
Most problems on z/OS are fixed while the system runs. An IPL affects every application on the system and is a last resort for recovery.
In a sysplex, an IPL of one system shifts work to the others. Check they have capacity before you start.
What you will see at work
- Operators perform the IPL; systems programmers prepare the configuration and are on call while it happens.
- Many sites IPL each system on a regular cycle, for example monthly, to bring in maintenance in a controlled way.
- Mainframe Path's Command Center includes a full IPL drill, from the HMC Load to the final checks.
Key terms
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