Larger log Relative Byte Address

This function improves the availability of the queue manager by increasing the period of time before you have to reset the log.

Recovery data is written to the log so that persistent messages are available when the queue manager is restarted. The term log Relative Byte Address (log RBA) is used to refer to the location of data as an offset from the beginning of the log.

Before IBM MQ Version 8.0, the 6 byte log RBA could address up to 256 terabytes of data. Before this quantity of log records has been written, you have to reset the queue manager's log by following the procedure documented in Resetting the queue manager's log.

Resetting the logs of queue managers is not a quick process, and can require an extended outage, due to the need to reset the page sets as part of the process. For a high use queue manager this operation might typically be done once a year.

From IBM MQ Version 8.0, the log RBA can be 8 bytes long and the queue manager can now address over 64,000 times as much data (16 exabytes) before the log RBA needs to be reset. The impact of using the larger log RBA is that the size of the log data written increases by a few bytes.


When is this function enabled?

We can enable this function at any time, although ideally we should plan for the change and explicitly enable it at a time to suit your enterprise, before the current log RBA approaches the end of the 6 byte log RBA range. See Plan to increase the maximum addressable log range for guidance on planning the migration to 8 byte log RBAs.

Important: Before we can use this capability in a queue sharing group, all queue managers in the queue sharing group must be at one of the following levels:

  • At Version 9.0.n CD, Version 9.1.0 LTS, or later
  • At Version 9.0.0 and have been started with OPMODE=(NEWFUNC,800), or OPMODE=(NEWFUNC,900)

If the current log RBA is approaching the end of the log RBA range, consider converting the queue manager to use an 8 byte log RBA rather than resetting the queue manager's log. Converting a queue manager to use 8 byte log RBAs requires a shorter outage than resetting the log, and significantly increases the period of time before you have to reset the log.

Message CSQJ034I, issued during queue manager initialization, indicates the end of the log RBA range for the queue manager as configured, and can be used to determine whether 6 byte or 8 byte log RBAs are in use.


How is this function enabled?

8 byte log RBA is enabled by starting the queue manager with a version 2 format BSDS. In summary, this is achieved by:
  1. Ensuring that all queue managers in the queue sharing group meet the requirements for enabling 8 byte log RBA
  2. Shutting down the queue manager cleanly
  3. Running the BSDS conversion utility to create a copy of the BSDS in version 2 format.
  4. Restarting the queue manager with the converted BSDS.

Once a queue manager has been converted to use 8 byte log RBAs, it cannot go back to using 6 byte log RBA.

See Implement the larger log Relative Byte Address for the detailed procedure on how to enable 8 byte log RBAs.

Parent topic: Logging in IBM MQ for z/OS


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