Redolog File Multiplexing

To protect against failure of the redo logs, Oracle allows redo logs to be multiplexed. With multiplexed redo logs two or more identical copies can be placed in separate locations. The log writer process (LGWR) writes the same redo information to each multiplexed log.

Multiplexing uses groups of redo log files. A group contains a redo log and all of its multiplexed copies. Each copy or member is identical. It is required that there are at least two groups and in the case of multiple members per group that all members be the same size. It is recommend but not required that each group has the same number of members.

Multiplexing Steps

Check redolog file members

SQL> select member from v$logfile;

To check redolog group info,status and size

SQL> select group#,members,status,sum(bytes/1024/1024) from v$log group by group#,members,status;

To add a redolog file group

SQL> alter database add logfile group 4 ('/u01/prod/redo04a.log','/u02/prod/redo04b.log') size 50m; 

To add a redolog member

SQL> alter database add logfile member '/u02/prod/redo01b.log' to group 1; 

To drop a redolog group

SQL> alter database drop logfile group 4; 

To drop a redolog member

SQL> alter database drop logfile member '/u02/prod/redo01b.log';

Rename / Relocate Redolog File

We need to shutdown the DB in order to perform this activity

SQL> shutdown immediate; 
SQL> ! cp /u02/prod/redo01.log /u02/prod/redo01a.log 
(If relocating, use the source and destination paths) 

SQL> startup mount 
SQL> alter database rename file '/u02/prod/redo01.log' to '/u02/prod/redo01a.log';

The above command will make server process to update the controlfile with new file name

SQL> alter database open;