ARCHIVELOG MODE ENABLE AND DISABLE

Archive log files are the offline copies of the online redolog files to enable recovery incase of failures and media recovery.

An Oracle database can run in one of two modes. By default, the database is created in NOARCHIVELOG mode. When in NOARCHIVELOG mode the database runs normally, but there is no capacity to perform any type of point in time recovery operations or online backups. Thus, you have to shutdown the database to back it up, and when you recover the database you can only recover it to the point of the last backup. While this might be fine for a development environment, the big corporate types tend to frown when a weeks worth of current production accounting data is lost forever.

So, if you wish to avoid the wrath of the CEO and angry end-users, you will want to run Oracle in ARCHIVELOG mode. In ARCHIVELOG mode, the database will make copies of all online redo logs after they are filled. These copies are called archived redo logs. The archived redo logs are created via the ARCH process. The ARCH process copies the archived redo log files to one or more archive log destination directories
Enabling ARCHIVELOG Mode
Lets start by checking the current archive mode.
SQL> SELECT LOG_MODE FROM SYS.V$DATABASE;
LOG_MODE
------------
NOARCHIVELOG
else
SQL> archive log list;
Database log mode No Archive Mode
Automatic archival Disabled
Archive destination D:\oracle\ora92\RDBMS
Oldest online log sequence 0
Current log sequence 1
SQL>
SQL> startup mount
ORACLE instance started.

Total System Global Area 135338868 bytes
Fixed Size 453492 bytes
Variable Size 109051904 bytes
Database Buffers 25165824 bytes
Redo Buffers 667648 bytes
Database mounted.
SQL>
SQL> alter database archivelog;

Database altered.

SQL>
SQL> alter database open;

Database altered.

SQL>
SQL> archive log list;
Database log mode Archive Mode
Automatic archival Disabled
Archive destination D:\oracle\ora92\RDBMS
Oldest online log sequence 0
Next log sequence to archive 1
Current log sequence 1
SQL> alter system set log_archive_start=true scope=spfile;

System altered.

SQL> archive log list;
Database log mode Archive Mode
Automatic archival Disabled
Archive destination D:\oracle\ora92\RDBMS
Oldest online log sequence 0
Next log sequence to archive 1
Current log sequence 1
SQL> shu immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup
ORACLE instance started.

Total System Global Area 135338868 bytes
Fixed Size 453492 bytes
Variable Size 109051904 bytes
Database Buffers 25165824 bytes
Redo Buffers 667648 bytes
Database mounted.
Database opened.
SQL> archive log list;
Database log mode Archive Mode
Automatic archival Enabled
Archive destination D:\oracle\ora92\RDBMS
Oldest online log sequence 0
Next log sequence to archive 1
Current log sequence 1
SQL>
Learn more about managing archive redo logs in the Oracle Database Administrator's Guide:
http://download-west.oracle.com/docs/cd/B14117_01/server.101/b10739/archredo.htm