MQGET for SYSTEM.ADMIN.COMMAND.QUEUE failed with reason code 2071

Cause

Well designed MQ server applications typically assume a relatively small initial buffer size, and then heuristically tune the required buffer size based upon the sizes of the messages consumed. For example both the repository manager and the pubsub broker adopt such a scheme.

The MQ command server however allocates a fixed size buffer based upon the MAXMSGL defined for the SYSTEM.ADMIN.COMMAND.QUEUE and then issues it's MQGET's using this buffer size. Because the commands accepted by the command server are not intrinsically big, and the default MAXMSGL for SYSTEM.ADMIN.COMMAND.QUEUE is only 9000 bytes, then this simplistic scheme is generally adequate in this scenario.

However if a very large MAXMSGL is set for SYSTEM.ADMIN.COMMAND.QUEUE, then this would be a relatively inefficient design, as the command server would allocate a very large buffer and the queue manager would interpret this as a hint that very large messages were going to be processed.

If the max message length for the SYSTEM.ADMIN.COMMAND.QUEUE is increased, the command server allocates a larger buffer when reading messages from this queue. If it is significantly increased, say from 4 to 100 MB, the "data" ulimit for the process can be insufficient, causing it to crash.