Depending on the circumstances under which you want to start or restart z/OS®, you may use one of three types of initialization: Cold start, quick start, or warm start.

The primary difference between the three types is whether or not certain storage is treated.

Cold start

An IPL that loads (or reloads) the pageable link pack area (PLPA) and clears the virtual input/output (VIO) data set pages.

The first IPL after system installation is always a cold start because the PLPA is initially loaded. Subsequent IPLs are cold starts when the PLPA is reloaded, either to alter its contents or to restore its contents if they were lost. This is usually done when changes have been made to the LPA (for example, when a new SYSRES containing maintenance is being loaded).

VIO is a method of using memory to store small temporary data sets for rapid access. However, unlike a RAM disk on a PC, these are actually backed up to disk and so can be used as a restart point. Obviously there should not be too much data stored in this way, so the size is restricted.

Quick start

An IPL that does not reload the PLPA, but clears the VIO data set pages. (The system resets the page and segment tables to match the last-created PLPA.) This is usually done when there have been no changes to LPA, but VIO must be refreshed. This prevents the warm start of jobs that were using VIO data sets.

Warm start

An IPL that does not reload the PLPA, and preserves journaled VIO data set pages. This will allow jobs that were running at the time of the IPL to restart with their journaled VIO data sets.

Often, the preferred approach is to do a cold start IPL (specifying CLPA). The other options can be used, but extreme care must be taken to avoid unexpected change or backout of change. A warm start could be used when you have long-running jobs which you want to restart after IPL, but an alternative approach is to break down those jobs into smaller pieces which pass real data sets rather than use VIO. Modern disk controllers with large cache memory have reduced the need for VIO data to be kept for long periods.

Also, do not confuse a cold start IPL (CLPA would normally be used rather than the term "cold start") with a JES cold start. Cold starting JES is something that is done extremely rarely, if ever, on a production system, and totally destroys the existing data in JES.