DBA Admin06 - Archived Redo Log Files

Oracle Database lets you save filled groups of redo log files to one or more offline destinations, known collectively as the archived redo log. The process
of turning redo log files into archived redo log files is called archiving. This process is only possible if the database is running in ARCHIVELOG mode.
If you are multiplexing your redo log, and if group 1 contains identical member files a_log1 and b_log1, then the archiver process (ARCn) will archive one of these member files. Should a_log1 become corrupted, then ARCn can still archive the identical b_log1. The archived redo log contains a copy of every group created since you enabled archiving.
You can use archived redo logs for the following situations.
■ Recover a database
■ Update a standby database
■ Get information about the history of a database using the LogMiner utility

How to select between Archive and Non archivelog mode?

The choice of whether to enable the archiving of filled groups of redo log files depends on the availability and reliability requirements of the application running on the database. If you cannot afford to lose any data in your database in the event of a disk failure, use ARCHIVELOG mode. The archiving of filled redo log files can require you to perform extra administrative operations.
To restore a database operating in NOARCHIVELOG mode, you can use only whole database backups taken while the database is closed. Therefore, if you decide to operate a database in NOARCHIVELOG mode, take whole database backups at regular, frequent intervals.
A database backup, together with online and archived redo log files, guarantees that you can recover all committed transactions in the event of an operating system or disk failure. If you keep an archived log, you can use a backup taken while the database is open and in normal system use.

How to change Database to Archiving Mode

1. Shut down the database instance.
2. Back up the database.
3. Edit the initialization parameter file to include the initialization parameters that specify the destinations for the archived redo log files.
5. Change the database archiving mode. Then open the database for normal operations.
6. Shut down the database.
7. Back up the database.

The LOG_ARCHIVE_MAX_PROCESSES initialization parameter specifies the number of ARCn processes that the database initially invokes. The default is two processes. to avoid any runtime overhead of invoking additional ARCn processes, you can set the LOG_ARCHIVE_MAX_PROCESSES initialization parameter to specify up to ten ARCn processes to be started at instance startup.

Archive Destinations

You can choose whether to archive redo logs to a single destination or multiplex them. If you want to archive only to a single destination, you specify that destination in the LOG_ARCHIVE_DEST initialization parameter. If you want to multiplex the archived logs, you can choose whether to archive to up to ten locations (using the LOG_ARCHIVE_DEST_n parameters) or to archive only to a primary and secondary destination (using LOG_ARCHIVE_DEST and LOG_ARCHIVE_DUPLEX_DEST).

Archived Redo Logs Views
Several dynamic performance views contain useful information about archived redo
logs, as summarized in the following table.

V$DATABASE Shows if the database is in ARCHIVELOG or NOARCHIVELOG mode and if MANUAL (archiving mode) has been specified.
V$ARCHIVED_LOG Displays historical archived log information from the control file. If you use a recovery catalog, the RC_ARCHIVED_LOG view contains similar information.
V$ARCHIVE_DEST Describes the current instance, all archive destinations, and the current value, mode, and status of these destinations.
V$ARCHIVE_PROCESSES Displays information about the state of the various archive processes for an instance.
V$BACKUP_REDOLOG Contains information about any backups of archived logs. If you use a recovery catalog, the RC_BACKUP_REDOLOG contains similar information.
V$LOG Displays all redo log groups for the database and indicates which need to be archived.
V$LOG_HISTORY Contains log history information such as which logs have been archived and the SCN range for each archived log.

The SQL*Plus command ARCHIVE LOG LIST displays archiving information for the connected instance. For example:

Database log mode Archive Mode
Automatic archival Enabled
Archive destination D:\oracle\oradata\IDDB2\archive
Oldest online log sequence 11160
Next log sequence to archive 11163
Current log sequence 11163

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