Page sets
A page set is a VSAM linear data set that has been specially formatted to be used by WebSphere 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. Shared queues are discussed in Shared queues and queue-sharing groups, and global definitions are discussed in Private and global definitions.
WebSphere 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.
WebSphere MQ uses page set zero (PSID=00) to store object definitions and other important information relevant to the queue manager. For normal operation of WebSphere 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 your system, you should also separate short-lived messages from long-lived messages by placing them on different page sets.
You must format page sets, and WebSphere MQ provides a FORMAT utility for this. This is described in the WebSphere MQ for z/OS System Administration Guide. Page sets must also be defined to the WebSphere MQ subsystem, this is described in the WebSphere MQ for z/OS System Setup Guide.
WebSphere MQ for z/OS can be configured to expand a page set dynamically if it becomes full. WebSphere MQ continues to expand the page set if required until 119 logical extents exist, provided that there is sufficient disk storage space available. The extents can span volumes if the linear data set is defined in this way, however, WebSphere MQ cannot expand the page sets beyond 64 GB.
We cannot use page sets from one WebSphere MQ queue manager on a different WebSphere MQ queue manager, or change the queue manager name. If you want to transfer the data from one queue manager to another, unload all the objects and messages from the first queue manager and reload them onto another.
In previous releases, page sets were limited to 4 GB. It is not possible to modify such existing page sets to be larger than 4 GB. Instead, create new page sets with extended addressability and extended format attributes.
It is not possible to use page sets greater than 4 GB in a queue manager running a release prior to V6. During the migration period, when it is likely that you might need to fall back to a previous release of code:
- 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.
A procedure for migrating existing page sets to be capable of expanding beyond 4 GB is documented in the WebSphere MQ for z/OS System Administration Guide (see Chapter 10 'Managing page sets', heading '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 (with the exception of page set zero). The DEFINE PSID command can run after the queue manager restart has completed, only if the command contains the DSN keyword.