How the logs are written

WebSphere MQ writes each log record to a DASD data set called the active log. When the active log is full, WebSphere MQ copies its contents to a DASD or tape data set called the archive log. This process is called off-loading.

Figure 10 illustrates the process of logging. Log records typically go through the following cycle:

  1. WebSphere MQ notes changes to data and significant events in recovery log records.

  2. WebSphere MQ processes recovery log records and breaks them into segments, if necessary.

  3. Log records are placed sequentially in output log buffers, which are formatted as VSAM CIs. Each log record is identified by a relative byte address in the range zero through 248-1.

  4. The CIs are written to a set of predefined DASD active log data sets, which are used sequentially and recycled.

  5. If archiving is active, as each active log data set becomes full, its contents are automatically off-loaded to a new archive log data set.

Figure 10. The logging process