REDO LOG FILE

Redo log files are filled with redo records. A redo record also called a redo entry, is made up of a group of change vectors, each of which is a description of a change made to a single block in the database. For example, if you change a salary value in an employee table, you generate a redo record containing change vectors that describe changes to the data segment block for the table, the undo segment data block, and the transaction table of the undo segments.

Redo entries record data that you can use to reconstruct all changes made to the database, including the undo segments. Therefore, the redo log also protects rollback data. When you recover the database using redo data, the database reads the change vectors in the redo records and applies the changes to the relevant blocks.

SQL> select * from v$log;

SQL> select group#,status,sequence# from v$log;

    GROUP# STATUS            SEQUENCE#
---------- ---------------- ----------
         1 CURRENT                  91
         2 INACTIVE                 89
         3 INACTIVE                 90

SQL> select * from v$logfile;

Active (Current) and Inactive Redo Log Files

Oracle Database uses only one redo log file at a time to store redo records written from the redo log buffer. The redo log file that LGWR is actively writing to is called the current redo log file.

Redo log files that are required for instance recovery are called active redo log files. Redo log files that are no longer required for instance recovery are called inactive redo log files.

If you have enabled archiving (the database is in ARCHIVELOG mode), then the database cannot reuse or overwrite an active online log file until one of the archiver background processes (ARCn) has archived its contents. If archiving is disabled (the database is in NOARCHIVELOG mode), then when the last redo log file is full, LGWR continues by overwriting the first available active file.

Add New redo log file

SQL> alter database add logfile group 4 '/u01/app/oracle/oradata/TESTDB/redo04.dbf' 
size 100m;

Database altered.

Add Member to an existing group

SQL> alter database add logfile member '/u01/app/oracle/oradata/TESTDB/redo04b.log' 
to group 4;

Database altered.

Drop member

SQL> alter database drop logfile member '/u01/app/oracle/oradata/TESTDB/redo04b.log';

Database altered.

Drop Logfile Group

SQL> alter database drop logfile group 4;

Database altered.


SQL> select group#,status from v$log;

    GROUP#                STATUS
----------            ----------------
         1               CURRENT
         2               INACTIVE
         3               INACTIVE

Adding Two  member

SQL> alter database add logfile group 4 ('/u01/app/oracle/oradata/TESTDB/redo04a.log',
'/u01/app/oracle/oradata/TESTDB/redo04C.log') size 100m;

Database altered.

SQL> select group#,member from v$logfile;

    GROUP#         MEMBER
----------       ---------------------------                       
         4    /u01/app/oracle/oradata/TESTDB/redo04a.log

         4     /u01/app/oracle/oradata/TESTDB/redo04C.log

Log Switches and Log Sequence Numbers

A log switch is a point at which the database stops writing to one redo log file and begins writing to another. Normally, a log switch occurs when the current redo log file is completely filled and writing must continue to the next redo log file. However, you can configure log switches to occur at regular intervals, regardless of whether the current redo log file is completely filled. You can also force log switches manually.

Oracle Database assigns each redo log file a new log sequence number every time a log switch occurs and LGWR begins writing to it. When the database archives redo log files, the archived log retains its log sequence number. A redo log file that is cycled back for use is given the next available log sequence number.

Each online or archived redo log file is uniquely identified by its log sequence number. During a crash, instance, or media recovery, the database properly applies to redo log files in ascending order by using the log sequence number of the necessary archived and redo log files.

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