MQ - z/OS - Logging
Any changes to objects held on page sets, and operations on persistent messages, are recorded as log records. These log records are written to a log data set called the active log. The name and size of the active log data set is held in a data set called the bootstrap data set (BSDS).
When the active log data set fills up, the queue manager switches to another log data set so that logging can continue, and copies the content of the full active log data set to an archive log data set. Information about these actions, including the name of the archive log data set, is held in the bootstrap data set. Conceptually, there is a ring of active log data sets that the queue manager cycles through; when an active log is filled, the log data is off-loaded to an archive log, and the active log data set is available for reuse.
The log and bootstrap data sets are discussed in Logging.