Understanding the log range required for recovery

During restart, the range of log data which must be read is dependent on many factors:

• At the time of an abnormal termination, there are typically many incomplete units of work in the system. As described earlier, restart processing will bring the system to a state of consistency, which may involve backing out inflight units of work, or recovering locks on indoubt units of work. Unit of work recovery requires that all unit of work log records for inflight, in-backout, and in-doubt units of work are available. WebSphere MQ V6 will 'shunt' old units of work, so that unit of work recovery can be performed using a much smaller range of log data.

• At the time of an abnormal termination, there are typically many persistent updates which are only held in the buffer pool cache. They have not yet been written to disk. These changes must be read from the log, and reapplied to the data held in page sets. Page set recovery RBAs in the checkpoint describe the lowest log RBA required for updating the page sets to a consistent state.

• If old page sets have been introduced into the system, for example, a page set backup has been introduced to recover from a media failure, all the changes must be read from the log from the time the backup was taken. These changes are reapplied to the data held in the page set being recovered. Page set recovery RBAs held in page 0 of the page set describe the lowest log RBA required for media recovery of a page set.

• If using persistent messages on shared queues, a range of log data is required to recover CFSTRUCTs which are holding persistent messages. The earliest log data that would be required to perform a CFSTRUCT recovery, is from around the time of the old CFSTRUCT BACKUP.

During normal running, the DISPLAY USAGE TYPE(DATASET) command can be used to view the recovery log range associated with these factors (it is unable to provide information due to reintroducing old page sets, of course). A best practice would be to regularly monitor the values output from DISPLAY USAGE TYPE(DATASET) to identify any issues that might prolong a queue manager restart in the event of an abnormal termination.

In addition, the queue manager issues informational messages relating to these factors:

• CSQJ160I and CSQJ161I warn of long running units of work.

• CSQR026I and CSQR027I provide information about whether these long running units of work have been successfully shunted.

• CSQE040I and CSQE041E warn that structure backups are getting old, and consequently a RECOVER CFSTRUCT operation would take a long time.