Oracle maintains two important pointers that you should be aware of.
- One points to the tail of the log and
- the other points to the head of the log.
As changes are made to a database, new redo log entries are always placed at the head of the log. The Log Writer process
, on the other hand, always looks at the tail of the log.
The Log Writer's job is to continually check the tail of the log to see if there are any new redo log records, and then to write those records to the redo log files as quickly as possible. The following series of images illustrates this process:
Choosing the LGWR attribute indicates that the log writer process (LGWR) will concurrently create the archived redo logs as the online redo log is populated. Depending on the configuration, this may require the log writer process to also transmit redo log files to remote archival destinations. Choosing the ARCH attribute indicates that the (ARCn) archiver process will create archived redo logs on the primary database and also transmit redo logs for archival at specified destinations. This is the default setting. The LGWR and ARCH attributes are mutually exclusive. Therefore, you cannot specify the two attributes for the same destination.
The LGWR attribute can be specified for individual destinations and this allows you to specify that the log writer process writes to redo logs and archives for some destinations while the archiver process archives redo logs to other destinations.