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Managing Redo Logfiles in Oracle

Every Oracle database must have at least 2 redo logfile groups. Oracle writes all statements except, SELECT statement, to the logfiles. This is done because Oracle performs deferred batch writes i.e. it does write changes to disk per statement instead it performs write in batches. So in this case if a user updates a row, Oracle will change the row in db_buffer_cache and records the statement in the logfile and give the message to the user that  row is updated. Actually the row is not yet written back to the datafile but still it give the message to the user that row is updated. After 3 seconds the row is actually written to the datafile. This is known as deferred batch writes.

 
Since Oracle defers writing to the datafile there is chance of power failure or system crash before the row is written to the disk. That’s why Oracle writes the statement in redo logfile so that in case of power failure or system crash oracle can re-execute the statements next time when you open the database.

Adding a New Redo Logfile Group

To add a new Redo Logfile group to the database give the following command

SQL>alter database add logfile group 3  ‘/u01/oracle/ica/log3.ora’ size 10M;

Note: You can add groups to a database up to the MAXLOGFILES setting you have specified at the time of creating the database. If you want to change MAXLOGFILE setting you have to create a new controlfile.

Adding Members to an existing group

To add new member to an existing group give the following command

SQL>alter database add logfile member ‘/u01/oracle/ica/log11.ora’ to group 1;

Note: You can add members to a group up to the MAXLOGMEMBERS setting you have specified at the time of creating the database. If you want to change MAXLOGMEMBERS setting you have create a new controlfile

Important: Is it strongly recommended that you multiplex logfiles i.e. have at least two log members, one member in one disk and another in second disk, in a database.

Dropping Members from a group

You can drop member from a log group only if the group is having more than one member and if it is not the current group. If you want to drop members from the current group, force a log switch or wait so that log switch occurs and another group becomes current. To force a log switch give the following command

SQL>alter system switch logfile;

The following command can be used to drop a logfile member

SQL>alter database drop logfile member ‘/u01/oracle/ica/log11.ora’;

Note: When you drop logfiles the files are not deleted from the disk. You have to use O/S command to delete the files from disk.

Dropping Logfile Group

Similarly, you can also drop logfile group only if the database is having more than two groups and if it is not the current group.

SQL>alter database drop logfile group 3;

Note: When you drop logfiles the files are not deleted from the disk. You have to use O/S command to delete the files from disk.

Resizing Logfiles

You cannot resize logfiles. If you want to resize a logfile create a new logfile group with the new size and subsequently drop the old logfile group.

Renaming or Relocating Logfiles

To Rename or Relocate Logfiles perform the following steps

For Example, suppose you want to move a logfile from ‘/u01/oracle/ica/log1.ora’ to ‘/u02/oracle/ica/log1.ora’, then do the following

Steps

1. Shutdown the database

SQL>shutdown immediate; 

2. Move the logfile from Old location to new location using operating system command

$mv /u01/oracle/ica/log1.ora  /u02/oracle/ica/log1.ora

3. Start and mount the database

SQL>startup mount

4. Now give the following command to change the location in controlfile

SQL>alter database rename file ‘/u01/oracle/ica/log1.ora’ to ‘/u02/oracle/ica/log2.ora’;

5. Open the database

SQL>alter database open;

Clearing REDO LOGFILES

A redo log file might become corrupted while the database is open, and ultimately stop database activity because archiving cannot continue. In this situation the ALTER DATABASE CLEAR LOGFILE statement can be used reinitialize the file without shutting down the database.

The following statement clears the log files in redo log group number 3:

ALTER DATABASE CLEAR LOGFILE GROUP 3;

This statement overcomes two situations where dropping redo logs is not possible:

If there are only two log groups
The corrupt redo log file belongs to the current group
If the corrupt redo log file has not been archived, use the UNARCHIVED keyword in the statement.

ALTER DATABASE CLEAR UNARCHIVED LOGFILE GROUP 3;

This statement clears the corrupted redo logs and avoids archiving them. The cleared redo logs are available for use even though they were not archived.

If you clear a log file that is needed for recovery of a backup, then you can no longer recover from that backup. The database writes a message in the alert log describing the backups from which you cannot recover

Viewing Information About Logfiles

To See how many logfile groups are there and their status type the following query.

SQL>SELECT * FROM V$LOG;
GROUP# THREAD#   SEQ   BYTES  MEMBERS  ARC STATUS     FIRST_CHANGE# FIRST_TIM
------ ------- ----- -------  -------  --- ---------  ------------- ---------
     1       1 20605 1048576        1  YES ACTIVE          61515628 21-JUN-07
     2       1 20606 1048576        1  NO  CURRENT         41517595 21-JUN-07
     3       1 20603 1048576        1  YES INACTIVE        31511666 21-JUN-07
     4       1 20604 1048576        1  YES INACTIVE        21513647 21-JUN-07

To See how many members are there and where they are located give the following query

SQL>SELECT * FROM V$LOGFILE;

GROUP#   STATUS  MEMBER
------  -------  ----------------------------------
     1           /U01/ORACLE/ICA/LOG1.ORA
     2           /U01/ORACLE/ICA/LOG2.ORA

 

 

 


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