Archived Redo log File

 Archived Redo Log File :

• Filled online redo log files can be archived.
• There are two advantages in running the database in ARCIHIVELOG mode and archiving online redo log files :

- Recovery: A Database backup togather with online and archived redo log files can guarantee recovery of all committed transaction.

- Backup: This can be performed while the database is open

• By Default, The database is created in NOARCHIVELOG mode. 

One of the important decisions that a database administrator has to make is whether the database is configured to operate in ARCHIVELOG mode or in NOARCHIVELOG mode.

NOARCHIVELOG MODE :

In noarchivelog mode, the online redo log files are overwritten each time an online redo log file is filled, and log switches occur. LGWR does not overwrite an online redo log file group until the checkpoint for that group is completed.

ARCHIVELOG MODE :

If the database is configured to run in ARCHIVELOG mode, inactive groups of filled online redo log files must be archived. Because all changes made to the database are recorded in the online redo log files,  The Database administrator should take the physical backup of the archived online redo log file. Backup of the archived redo log file is used to recover the database without losing any committed data when Database recovery required.

There are two ways  in which online redo log files can be archived :

• Manually
• Automatically ( recommended method )

The LOG_ARCHIVE_START initialization parameter indicates whether archiving should be automatic or manual when the instance starts up.

• TRUE: TRUE indicates that archiving is automatic. ARCn initiates the archiving of the filled log group at every log switch.
• FALSE: The default value is FALSE, and it indicates that the DBA archives the filled online redo log files manually. The DBA must manually execute a command each time you want to archive an online redo log file. All or specific online redo log files can be archived manually.

Please review some important facts about Archive redo log files :

• Archiving accomplished automatically by ARCn
• Archiving accomplished manually through SQL statements
• When the online redo log file successfully archived below event occurs :

- An entry in the control file is made.

- Records: archive log name, log sequemce number, and high and low system change number (SCN)

• Filled online redo log files cannot be reused until: 

- A checkpoint has taken place

- File has been archived by ARCn

• Can be multiplexed
• Maintained by the DBA

Information about archive logs can be obtained from V$INSTANCE view.

SQL> SELECT archiver FROM V$INSTANCE;

ARCHIVE

-------

STARTED

The archiver parameter having a different status as below.

STOPPED: This status shows that the archiving has stopped working.

STARTED: This status shows that archiving is in progress and archiving is done by the Oracle Background Process ARCn.

FAILED: This status shows that the archiver failed to archive a log last time but will try again within 5 minutes.

Thank you !!