When you plan your queue manager backup strategy, a key consideration is retention of the correct amount of log data. The WebSphere MQ for z/OS System Setup Guide describes how to determine which log data sets are required, by reference to the system recovery RBA of the queue manager. WebSphere MQ determines the system recovery RBA using information about the following:
You must retain sufficient log data to be able to perform media recovery. Whilst the system recovery RBA increases over time, the amount of log data that must be retained only decreases when subsequent backups are taken. CF structure backups are managed by WebSphere MQ, and so are taken into account when reporting the system recovery RBA. This means that in practice, the amount of log data that must be retained only reduces when page set backups are taken.
Figure 23 shows an example of the backup activity on a queue manager that is a member of a queue-sharing group, how the recovery RBA varies with each backup, and how that affects the amount of log data that must be retained. In the example the queue manager uses local and shared resources: page sets, and two CF structures, STRUCTURE1 and STRUCTURE2.
This is what happens at each point in time:
The system recovery RBA of the queue manager is the lowest of the following:
The system recovery RBA for this point in time is given by messages issued by the DISPLAY USAGE command, which is part of the fuzzy backup process.
The amount of log data that must be retained is unchanged, because the same data as determined from the system recovery RBA at T1 is still required to recover using the page set backups taken at T1.
The system recovery RBA of the queue manager is the lowest of the following:
The system recovery RBA for this point in time is given by messages issued by the DISPLAY USAGE command, which is part of the fuzzy backup process.
You can now reduce the log data retained, as determined by this new system recovery RBA.
The creation of this CF structure backup has no impact on the amount of log data that must be retained.
The creation of this CF structure backup has no impact on amount of log data that must be retained.
The system recovery RBA of the queue manager is the lowest of the following:
The system recovery RBA for this point in time is given by messages issued by the DISPLAY USAGE command, which is part of the full backup process.
Again, the log data retained can be reduced, because the system recovery RBA associated with the full backup is more recent.
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