oracle Enabling ARCHIVELOG Mode

Enabling ARCHIVELOG Mode

   Most of the High Availability features of Oracle require you to enable ARCHIVELOG mode for your database.  When you enable this mode redo logs will be archived instead of overwritten.  The archivelogs are stored in a seperate place usually can backed up regularly by your standard filesystem backup system (NetBackup or whatever). Archive logs are utilized by RMAN, Data Guard, Flashback and many others.  
If your going to enable archivelog mode on a real database thats important to you, I would recommend shutting down the database and doing a cold backup just in case.  Keeping a "final noarchivelog mode backup" seems to be a good and excepted practice.  
Enabling archive mode is simple, just connect to your database in mounted but closed mode (startup mount) and alter the database.  But if you don't tune alittle you'll run into problems down the road, so lets specify some parameters too.  Namely, consider LOG_ARCHIVE_DEST.  
Lets start by checking the current archive mode.  

SQL> SELECT LOG_MODE FROM SYS.V$DATABASE;

LOG_MODE
------------
NOARCHIVELOG
  So we're in NOARCHIVELOG mode and we need to change.   We can use a database alter statement, but that won't be perminant, so lets just update the pfile directly. The pfile should be in either $ORACLE_BASE/admin/SID/pfile or $ORACLE_HOME/admin/SID/pfile.  I'll add the following lines to the end of the file:  

############################
# Archive Log Destinations -benr(10/15/04)
############################
log_archive_dest_1='location=/u02/oradata/cuddle/archive'
log_archive_start=TRUE
  Note that we're not actually required to specify the location of the log destination, but if you don't it'll end up  in strange places (in my test it went to $ORACLE_HOME/dbs making a mess). You can specify as many as 10 diffrent archive log destinations by using the paramters log_archive_dest_1 through log_archive_dest_10. Remember, if you run out of space in your archive log destination the database will shut down!     
Now we can startup the database in mount mode and put it in  archivelog mode.  

[oracle@vixen pfile]$sqlplus sys/passwd as sysdba;
SQL*Plus: Release 10.1.0.2.0 - Production on Fri Oct 15 16:00:58 2004
Copyright (c) 1982, 2004, Oracle.  All rights reserved.
Connected to an idle instance.

SQL> startup mount
ORACLE instance started.

Total System Global Area  184549376 bytes
Fixed Size                  1300928 bytes
Variable Size             157820480 bytes
Database Buffers           25165824 bytes
Redo Buffers                 262144 bytes
Database mounted.

SQL> alter database archivelog;
Database altered.

SQL> alter database open;
Database altered.
  You can see here that we put the database in ARCHIVELOG mode by using the SQL statement "alter database archivelog", but  Oracle won't let us do this unless the instance is mounted but not open.  To make the change we shutdown the instance, and then startup the instance again but this time with the "mount" option which will mount the instance but not open it.  Then we can enable ARCHIVELOG mode and open the database fully with the "alter database open" statement.  
There are several system views that can provide us with  information reguarding archives, such as:  

V$DATABASEIdentifies whether the database is in ARCHIVELOG or NOARCHIVELOG mode and whether MANUAL (archiving mode) has been specified. V$ARCHIVED_LOGDisplays historical archived log information from the control file. If you use a recovery catalog, the RC_ARCHIVED_LOG view contains similar information. V$ARCHIVE_DESTDescribes the current instance, all archive destinations, and the current value, mode, and status of these destinations. V$ARCHIVE_PROCESSESDisplays information about the state of the various archive processes for an instance. V$BACKUP_REDOLOGContains information about any backups of archived logs. If you use a recovery catalog, the RC_BACKUP_REDOLOG contains similar information. V$LOGDisplays all redo log groups for the database and indicates which need to be archived. V$LOG_HISTORYContains log history information such as which logs have been archived and the SCN range for each archived log.    Using these tables we can verify that we are infact in ARCHIVELOG mode:  

SQL> select log_mode from v$database;

LOG_MODE
------------
ARCHIVELOG

SQL> select DEST_NAME,STATUS,DESTINATION from V$ARCHIVE_DEST;
  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