Archivelog is an optional mode for the database. You can set it with the INIT.ORA parameter ARCHIVELOG or in the ALTER DATABASE statement. In this mode, the ARCH background process copies the contents of each redo log to an archive file at the time of a log switch. Be sure to allocate enough redo logs so that ARCH has sufficient time to copy the redo log to the archive file before the redo log is reused. Archivelog mode comes at a cost of increased overhead. The ARCH process reads from the most recently used redo log at the same time that the LGWR process is writing to the current redo log.