What is Redo Log Buffer in Oracle database ?

What is Redo Log Buffer in Oracle ?

Redo Log Buffer is a circular buffer in the SGA that holds information about changes made to the database where changes can be DML or DDL, etc. This information is stored in 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 used for database recovery, if necessary. Redo log files are filled with redo records. A redo record is also called a redo entry and is made up of a group of change vectors each of which is a description of a change made to a single block in the database. It contains both committed as well uncommitted information.

LGWR or Log Writer is the process which writes information from Redo Log Buffer to redo log file.

Whenever a transaction is committed, LGWR writes the transaction redo records from the redo log buffer of the SGA to a redo log file, and assigns a system change number (SCN) to identify the redo records for each committed transaction. Only when all redo records associated with a given transaction are safely on disk in the online logs is the user process notified that the transaction has been committed. 

We use the LOG_BUFFER initialization parameter to set the size in bytes of the redo log buffer, and it stays fixed for the duration of the instance. That is, we can’t adjust the redo log buffer size dynamically, unlike the other components of the SGA. The default setting is four times the maximum data block size for the host operating system.

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