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To protect against a failure involving the redo log itself, Oracle Database allows a multiplexed redo log, meaning that two or more identical copies of the redo log can be automatically maintained in separate locations. For the most benefit, these locations should be on separate disks. Even if all copies of the redo log are on the same disk, however, the redundancy can help protect against I/O errors, file corruption, and so on. When redo log files are multiplexed, LGWR concurrently writes the same redo log information to multiple identical redo log files, thereby eliminating a single point of redo log failure.

Multiplexing is implemented by creating groups of redo log files. A group consists of a redo log file and its multiplexed copies. Each identical copy is said to be a member of the group. Each number defines a redo log group. Such as group 1, group 2, and so on.  ( More information on finding online redolog files )

1. Adding A New redo log Group

sql> ALTER DATABASE ADD LOGFILE('/oracledata/test/redo3a.log','/test_multiplex/redo3b.log') SIZE 52429312;

The command above will add a new redo log group to your database. The group will have two members with size 52429312. A group number will automatically be assigned to your new group. If you want to set it explicitly you may use command below.

sql> ALTER DATABASE ADD LOGFILE THREAD 1 GROUP 15 ('/oracledata/test/redo3a.log','/test_multiplex/redo3b.log') SIZE 52429312;

In a RAC (Real Application Clusters) environment you have to specify which instance the redo log group will belong to by setting thread number. Every instance in a RAC database has a thread number. For ex: the command below will add the redo log group to instance with thread number 1.

sql> ALTER DATABASE ADD LOGFILE THREAD 1 GROUP 15 ('/oracledata/test/redo3a.log','/test_multiplex/redo3b.log') SIZE 52429312;

2. Dropping A redo log Group

Before dropping a redo log group, make sure that at the moment, logwriter process is not writing to the redo log file you plan to drop.

sql> SELECT GROUP#, THREAD#,ARCHIVED, STATUS FROM V$LOG ;
GROUP#  THREAD#  ARC STATUS
 1  1  YES INACTIVE
 2  1  NO CURRENT
 3  1  YES INACTIVE 

The value for "ARCHIVED" column must be "YES" and STATUS column must be "INACTIVE".

If the status is "CURRENT" it means that logwriter is currently using this group to store redo records. After the redo files in this group is filled it will switch to next group. You may wait till next log switch operation or you may manually trigger it by command below:

SQL> alter system switch logfile ;

If status is "ACTIVE" it means that there are some redo log records, which have not been applied to datafiles yet. The status is supposed to turn to INACTIVE soon after the next checkpoint operation. If you do not want to wait till then you may manually trigger checkpoint by command below.

sql> alter system checkpoint ;

If the status has turned to INACTIVE you may drop it by specifying the group number.

sql> ALTER DATABASE DROP LOGFILE GROUP 6;

This command will drop the redo log group, which is identified by number 6.

Logwriter writes to redo log files in a circular fashion. When a group is filled it moves to next group. After all groups in database are filled it returns back to first redo log group. Because of that, there has to be at least 2 groups in an instance. Oracle won't permit dropping a group if there are only two groups in the instance.

After dropping a group, you may delete relevant files from operating system.

3. Adding A New Member To An Existing Group

As mentioned at the beginning of article it is recommended to multiplex redo log members for fault tolerance. You can accomplish this by adding new members to an existing group.

sql> ALTER DATABASE ADD LOGFILE MEMBER '/u02/oradata/mydb/redo02.log' TO GROUP 2;

The above command adds a new redo log file to redo log group with number 2.

4. Dropping A Member From An Existing Group

As stated in subject 3 (Dropping A redo log Group) again the group, whose member you want to drop should be inactive.

sql> ALTER DATABASE DROP LOGFILE MEMBER '/u03/oradata/mydb/redo03.log'

5. Relocating A redo log Group

You may want to relocate your existing redo log group and change the path of redo log files. Here are the steps to accomplish it

Step 1: Shutdown database

sql> shutdown immediate ;

Step 2: Backup your existing control file and redo log file at operating system level.

Step 3: Move your redo log member to a new location. You may also change the file name if you want.

# mv /u02/oradata/mydb/redo02.log /u03/oradata/mydb/redo03.log

Step 4: Startup in nomount mode

sql> startup nomount;

Step 5: Path of redo log files is kept in control file. You have to update that record by command below:

sql> ALTER DATABASE RENAME FILE '/u02/oradata/mydb/redo02.log' TO '/u03/oradata/mydb/redo03.log'

Control file is modified and the path of redo log member is updated. Now you can open your database.

sql> alter database open;

     As you see this way of relocating online redo log files requires downtime.  You may also change the path of your redo log files by adding a new group at desired location and dropping the ones at former location. This approach requires no downtime.

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Posted by at 07:13
Tags :
checkpoint , control file , multiplexing , redolog , v$log

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