Page sets for IBM MQ for z/OS

Use this topic to understand how IBM MQ for z/OS uses pages sets to store messages.

A page set is a VSAM linear data set that has been specially formatted to be used by IBM MQ. Page sets are used to store most messages and object definitions.

The exceptions to this are global definitions, which are stored in a shared repository on Db2, and the messages on shared queues. These are not stored on the queue manager page sets. For more information about shared queues, see Shared queues and queue sharing groups, and for more information about global definitions, see Private and global definitions.

IBM MQ page sets can be up to 64 GB in size. Each page set is identified by a page set identifier (PSID), an integer in the range 00 through 99. Each queue manager must have its own page sets.

IBM MQ uses page set zero (PSID=00) to store object definitions and other important information relevant to the queue manager. For normal operation of IBM MQ it is essential that page set zero does not become full, so do not use it to store messages.

To improve the performance of our system, we should also separate short-lived messages from long-lived messages by placing them on different page sets.

We must format page sets, and IBM MQ provides a FORMAT utility for this; see Formatting page sets (FORMAT). Page sets must also be defined to the IBM MQ subsystem.

IBM MQ for z/OS can be configured to expand a page set dynamically if it becomes full. IBM MQ continues to expand the page set if required until 119 logical extents exist, if there is sufficient disk storage space available. The extents can span volumes if the linear data set is defined in this way, however, IBM MQ cannot expand the page sets beyond 64 GB.

We cannot use page sets from one IBM MQ queue manager on a different IBM MQ queue manager, or change the queue manager name. If you want to transfer the data from one queue manager to another, we must unload all the objects and messages from the first queue manager and reload them onto another.

• Do not change page set 0 to be greater than 4 GB.
• Other page sets greater than 4 GB will be left offline when restarting a queue manager with a previous release.

For further information about migrating existing page sets capable of expanding beyond 4 GB, see Defining a page set to be larger than 4 GB.

It is possible for an administrator to dynamically add page sets to a running queue manager, or remove page sets from a running queue manager (except for page set zero). The DEFINE PSID command can run after the queue manager restart has completed, only if the command contains the DSN keyword.