Media images

A WebSphere MQ object of long duration can represent a large number of journal entries, going back to the point at which it was created. To avoid this overhead, WebSphere MQ for iSeries has the concept of a media image of an object.

This media image is a complete copy of the WebSphere MQ object recorded in the journal. If an image of an object is taken, the object can be rebuilt by replaying journal entries from this image onwards. The entry in the journal that represents the replay point for each WebSphere MQ object is referred to as its media recovery entry. WebSphere MQ keeps track of the:

• Media recovery entry for each queue manager object.

• Oldest entry from within this set (see error message AMQ7462 in WebSphere MQ Messages for details.

Images of the *CTLG object and the *MQM object are taken regularly because these objects are crucial to queue manager restart.

Images of other objects are taken when convenient. By default, images of all objects are taken when a queue manager is shut down using ENDMQM with parameter ENDCCTJOB(*YES). This operation can take a considerable amount of time for very large queue managers. If we need to shut down quickly, specify parameter RCDMQMIMG(*NO) with ENDCCTJOB(*YES). In such cases, you are recommended to record a complete media image in the journals after the queue manager has been restarted, using the following command:

RCDMQMIMG OBJ(*ALL) OBJTYPE(*ALL) MQMNAME(<Q_MGR_NAME>)

WebSphere MQ automatically records an image of an object, if it finds a convenient point at which an object can be compactly described by a small entry in the journal. However, this might never happen for some objects, for example, queues that consistently contain large numbers of messages.

Rather than allow the date of the oldest media recovery entry to continue for an unnecessarily long period, use the WebSphere MQ command

RCDMQMIMG, which enables you to take an image of selected objects manually.