Media images on IBM i

On IBM i, a media image is a complete copy of an IBM MQ object that is recorded in the journal. Some corrupt or damaged objects can be automatically recovered from their media image.

An IBM 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, IBM MQ for IBM i has the concept of a media image of an object.

This media image is a complete copy of the IBM 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 IBM MQ object is referred to as its media recovery entry. IBM MQ keeps track of the:

• Media recovery entry for each queue manager object.
• Oldest entry from within this set (see error message AMQ7462 in Journal management on IBM i 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 the ENDMQM command with parameter ENDCCTJOB(*YES). This operation can take a considerable amount of time for very large queue managers. For to shut down quickly, specify parameter RCDMQMIMG(*NO) with ENDCCTJOB(*YES). In such cases, we 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)

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

Recovery from media images

IBM MQ automatically recovers some objects from their media image if it is found that they are corrupt or damaged. In particular, this applies to the special *MQM and *CTLG objects as part of the normal queue manager startup. If any syncpoint transaction was incomplete at the time of the last shutdown of the queue manager, any queue affected is also recovered automatically, in order to complete the startup operation.

We must recover other objects manually, using the IBM MQ command RCRMQMOBJ. This command replays the entries in the journal to re-create the IBM MQ object. Should an IBM MQ object become damaged, the only valid actions are to delete it or re-create it by this method. Note, however, that nonpersistent messages cannot be recovered in this fashion.

Parent topic: Queue manager journals on IBM i