How Redo Logs Work

The online redo log of a database consists of two or more online redo log files. Oracle needs a minimum of two files to guarantee that one is always available for writing while the other is being archived.The Oracle server sequentially records all changes made to the database in the redo log buffer. The redo entries are written from the redo log buffer to one of the online redo log groups called the current online redo log group by the LGWR process. LGWR writes under the following situations:

• When a transaction commits
• When the redo log buffer becomes one-third full
• When there is more than a megabyte of changed records in the redo log buffer
• Before the DBWn writes modified blocks in the database buffer cache to the data files

LGWR writes to online redo log files in a circular fashion.Each redo log file group is identified by a log sequence number that is overwritten when log is reused.LGWR writes on the online redo log files continueslly. When the current online redo log grooup is filled log writer starts writing to the next group. When the last online redo log file is filled, LGWR returns to the first online redo log group and starts writing again.

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