Log data

The log can contain up to 140 million million (1.4*10¹⁴) bytes. Each byte can be addressed by its offset from the beginning of the log, and that offset is known as its relative byte address (RBA).

The RBA is referenced by a 6-byte field giving a total addressable range of 2⁴⁸ bytes. However, when WebSphere MQ detects that the used range is beyond
x'700 000 000 000' (that is, more than 1.2*10¹⁴ bytes of the log have been used), messages CSQI045, CSQI046 and CSQI047 are issued, warning you to reset the log RBA. Do not allow the log RBA to go beyond 2⁴⁷ bytes (x'800 000 000 000').

The log is made up of log records, each of which is a set of log data treated as a single unit. A log record is identified either by the RBA of the first byte of its header, or by its log record sequence number (LRSN). The RBA or LRSN uniquely identifies a record that starts at a particular point in the log.

Whether you use the RBA or LRSN to identify log points depends on whether you are using queue-sharing groups. In a queue-sharing environment, we cannot use the relative byte address to uniquely identify a log point, because multiple queue managers can update the same queue at the same time, and each has its own log. To solve this, the log record sequence number is derived from a timestamp value, and does not necessarily represent the physical displacement of the log record within the log.

Each log record has a header that gives its type, the WebSphere MQ subcomponent that made the record, and, for unit of recovery records, a unit of recovery identifier.

There are four types of log record, described under the following headings:

• Unit-of-recovery log records

• Checkpoint records

• Page set control records

• CF structure backup records