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Redo Log Buffer Tips

Oracle Tips by Burleson Consulting

The Redo Log Buffer

A log buffer is a circular buffer in the SGA that holds information about changes made to the database. This information is stored in the redo entries. Redo entries contain the information necessary to reconstruct or redo changes made to the database by insert, update, delete, create, alter, or drop operations. Redo entries are primarily used for database recovery as necessary.

The server processes generate redo data into the log buffer as they make changes to the data blocks in the buffer. LGWR subsequently writes entries from the redo log buffer to the online redo log.

The redo log buffer is a RAM area (defined by the initialization parameter log_buffer) that works to save changes to data, in case something fails and Oracle has to put it back into its original state (a ?rollback?).  When Oracle SQL updates a table (a process called Data Manipulation Language, or DML), redo images are created and stored in the redo log buffer. Since RAM is faster than disk, this makes the storage of redo very fast.

Also see my notes on sizing the redo log buffer.

The Oracle redo log buffer provides the following functions within the Oracle SGA:

  • Serves for assistance with database recovery tasks
  • Records all changes made to database blocks
  • Places changes recorded to redo entries for redo logs

The database initialization parameter log_buffer defines the default size of the redo log buffer within Oracle 11g.

Oracle will eventually flush the redo log buffer to disk. This can happen in a number of special cases, but what?s really important is that Oracle guarantees that the redo log buffer will be flushed to disk after a commit operation occurs. When you make changes in the database you must commit them to make them permanent and visible to other users.

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Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

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