Archivelog Mode

We can see whether archivelog mode is enabled (works on just about all versions of oracle database:

[oracle@myserver]$ sqlplus / as sysdba

SQL*Plus: Release 11.2.0.1.0 Production on Thu Apr 8 12:02:52 2010
Copyright (c) 1982, 2009, Oracle.  All rights reserved.

Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options

SQL> archive log list
Database log mode              No Archive Mode
Automatic archival             Disabled
Archive destination            USE_DB_RECOVERY_FILE_DEST
Oldest online log sequence     62
Current log sequence           64
SQL>

In this database, the archivelog destination in this database is currently set to use the DB_RECOVERY_FILE_DEST parameter. Note, yours may be pointing to another location:

SQL> show parameter db_recovery_file_dest
NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
db_recovery_file_dest                string      /oracle/archivelogs
db_recovery_file_dest_size           big integer 1000M
SQL>

ENABLING ARCHIVELOG MODE:

SQL> shutdown immediate
...
SQL> startup mount
...
SQL> alter database archivelog;

Database altered.

SQL> alter database open;

Database altered.

SQL> archive log list
Database log mode              Archive Mode
Automatic archival             Enaabled
Archive destination            USE_DB_RECOVERY_FILE_DEST
Oldest online log sequence     62
Current log sequence           64
SQL>

Now, switch the logfile, and an archivelog will be written:

SQL> alter system switch logfile;

System altered.

SQL>!ls /oracle/archivelogs/
1_01_23444412.dbf

SQL>

Recap: Comands to enable:

[oracle@myserver]$ sqlplus / as sysdba
SQL> shutdown immediate
SQL> startup mount
SQL> alter database archivelog;
SQL> alter database open;
SQL>
SQL> archive log list 
Database log mode Archive Mode 
Automatic archival Enabled 
Archive destination USE_DB_RECOVERY_FILE_DEST 
Oldest online log sequence 62 
Current log sequence 64 
SQL>

DISABLING ARCHIVELOG MOD is basically the same commands in reverse:

[oracle@myserver]$ sqlplus / as sysdba
SQL> shutdown immediate
SQL> startup mount
SQL> alter database noarchivelog;
SQL> alter database open;
SQL>
SQL> archive log list 
Database log mode No Archive Mode 
Automatic archival Disabled 
Archive destination USE_DB_RECOVERY_FILE_DEST 
Oldest online log sequence 62 
Current log sequence 64 
SQL>