The instance treats the online redo log groups as a circular buffer in
which to store transaction information, filling one group and then moving on to
the next. After all groups have been written to, the instance begins
overwriting information in the first log group.
To configure your database for maximum recoverability, you must instruct
the database to make a copy of the online redo log group before allowing it to
be overwritten. These copies are known as archived logs.
To facilitate the creation of archive log files:
1. Specify a naming convention
for your archive logs.
2. Specify a destination or
destinations for storing your archive logs. One of the destinations is probably
your flash recovery area.
3. Place the database in ARCHIVELOG
mode.
Note: Steps 1
and 2 are not necessary if you are using a flash recovery area.
The destination must exist before placing the database in ARCHIVELOG
mode. When a directory is specified as a destination, there should be a slash
at the end of the directory name.
Archive
Log File: Naming and Destinations
To
configure archive log file names and destinations, select Enterprise Manager
> Availability > Configure Recovery Settings.
Each
archive log file must have a unique name to avoid overwriting older log files. To help create unique file
names, Oracle Database 11g allows several wildcard characters in the
name format:
•
%s: Includes the log sequence number as
part of the file name
•
%t: Includes the thread number as part
of the file name
•
%r: Includes the resetlogs ID to ensure
that the archive log file name remains unique (even after
certain advanced recovery techniques that reset log sequence numbers)
certain advanced recovery techniques that reset log sequence numbers)
•
%d: Includes the database ID as part of
the file name
The format must
include %s, %t, and %r. The use of %d is optional, but it must be included if
multiple databases share the same archive log destination.
Archive log
files can be written to as many as ten different destinations. Destinations may
be local (a directory) or remote (an Oracle Net alias for a standby database).
The default
destination (number 10) sends archive log files to a location determined by the
DB_RECOVERY_FILE_DEST initialization parameter. DB_RECOVERY_FILE_DEST is also
known as the RECOVERY AREA parameter. This destination is visible at the bottom
of the Recovery Settings properties page as Flash Recovery Area Location.
Note: If you do not want archives sent to
this location, delete USE_DB_RECOVERY_FILE_DEST.
To change
recovery settings, you must be connected as SYSDBA or SYSOPER.
Enabling
ARCHIVELOG Mode
1. In Enterprise Manager, select
Availability > Configure Recovery Settings > ARCHIVELOG Mode. The
equivalent SQL command is:
SQL> ALTER DATABASE ARCHIVELOG;
This command can be issued only while
the database is in the MOUNT state. The instance must therefore be restarted to
complete this last step.
2. In Enterprise Manager, you are prompted
for operating system and database credentials during the restart of the
database. The database credentials must be for a user with the SYSDBA
privileges.
3. After the instance is restarted, the
changes that you have made to the archive processes, log format, and log
destinations are in effect. In SQL*Plus, you can see them with the ARCHIVE LOG
LIST command.
4. Back up your database after switching to ARCHIVELOG
mode because your database is only recoverable from the last backup taken in
that mode.
With the
database in NOARCHIVELOG mode (the default), recovery is possible only until
the time of the last backup. All transactions made after that backup are lost.
In ARCHIVELOG
mode, recovery is possible until the time of the last commit. Most production
databases are run in ARCHIVELOG mode.
How to
manually turn on Archive Log
