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All About REDO LOG


ADD and DROP online redo log members and groups EASY WAY


Redo log Files: The redo log files record all changes made to any data in the database buffer cache except direct writes.
Redo log files are used in instance failure to recover committed data that has not been written to the data files. The redo log files are only used for database recovery.

Online Redo Log Groups
These are the sets of identical copies of online redo log files. The background process LGWR concurrently writes the same information to all online redo log files in a group if a group has more than one log file. The Oracle server needs a minimum of two online redo log file groups for the normal operation of a database. Although oracle suggests keeping three groups.

Online Redo Log Members
These are the members of online redo log groups. Each member in a group has identical log sequence number and of the same size. The log sequence number is assigned each time the server starts writing to a log group to identify each redo log file uniquely. The current log sequence number is stored in the control file and also in the header of all data files.

Obtaining Information about Groups and Members: 

The following query returns information about the online redo log file from the control file:
SQL> select group#, sequence#, bytes, members, status from v$log;

The following query returns information about all members of a group:
SQL > select * from v$logfile;

How to “ADD” Online Redo Log Groups : Adding groups can solve availability problems. To create a new group of online redo log files use the following command:

ALTER DATABASE ADD LOGFILE (' DATAFILE_LOCATION/redo03.log') size 1M;
OR 
ALTER DATABASE ADD LOGFILE GROUP 6(' DATAFILE_LOCATION /redo06.log')SIZE 50M;


How to “ADD”  Online Redo Log Members: Add new member to an existing redo log file group using the following command:
ALTER DATABASE ADD LOGFILE MEMBER ‘ DATAFILE_LOCATION /redo03.log' TO GROUP 1,
' DATAFILE_LOCATION /redo03.log' TO GROUP 2;

How to “DROP” Online Redo Log Groups : To drop a group of online redo log files use the following command:
ALTER DATABASE DROP LOGFILE GROUP 3;

How to “DROP” Online Redo Log Members: To drop a member of an online redo log group use the following command:
ALTER DATABASE DROP LOGFILE MEMBER
‘ DATAFILE_LOCATION /redo03.log'; 

Happy to Help.

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