What Is The Archived Redo Log?

Oracle Database lets you save filled groups of redo log files to one or more offline
destinations, known collectively as the archived redo log, or more simply the archive
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. You can choose automatic or manual archiving.
An archived redo log file is a copy of one of the filled members of a redo log group. It
includes the redo entries and the unique log sequence number of the identical member
of the redo log group. For example, 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.
When the database is running in ARCHIVELOG mode, the log writer process (LGWR)
cannot reuse and hence overwrite a redo log group until it has been archived. The
background process ARCn automates archiving operations when automatic archiving is enabled. The database starts multiple archive  processes as needed to ensure that the
archiving of filled redo logs does not fall behind.