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Lesson 2Overview of redo log processing
ObjectiveUnderstand redo log functions.

Oracle Redo Logs

One of the foremost features of Oracle is its ability to recover data after a disk failure. Whenever a failure occurs, the DBA can direct Oracle to use the redo logs to apply after-images of all database changes. When an Oracle database is rolled-forward, the redo logs are used to apply the row after-images to the database.
The following Slide Show will show you how redo logs travel through several stages on their way to the archived redo log file system.


  1. Redo entries are written to the redo log buffer. When a transaction issues a COMMIT or reaches a normal end of task, the entries are written to the online redo log files. The LGWR process takes the images from the log buffer and places them into the online redo log files.
  2. When Oracle switches redo log files, the old redo log file is copied to an archived redo log filesystem by the ARCH process. The redo log file can then be overwritten by Oracle.
  3. As the archived red log file system becomes full, a user-written process is invoked to copy the redo log files to tape for long-term storage.


Oracle Redo Log

There are several Oracle processes that assist with processing redo logs. These include:
ARCOMON or Archive monitor process This is a process on UNIX that monitors the archive process and writes the redo logs to the archives.
ARCH or Archiver Process This process is only active if archive logging is in effect. This process writes the redo log data files that are filled into the archive log data files.
LGWR or Log Writer process This process transfers data from the redo log buffers to the redo log database files.

Now that we have reviewed the basics, let us move on to look at the performance and tuning implications of the redo logs.

Redo Log Process

Before moving on to the next lesson, click the link below to read about the redo log function.
Redo Log Process