ARCN (Archiver Process)

The archiver process (ARCn) copies redo log files to a designated storage device after a log switch has occurred. ARCn processes are present only when the database is in ARCHIVELOG mode, and automatic archiving is enabled ARCn Responsible for copying online redo log to archival storage before being reused. Runs only when database is in Archivelog Mode.When a Redo Log File fills up, Oracle switches to the next Redo Log File.

• If all Redo Log Files fill up, then Oracle switches back to the first one and uses them in a round-robin fashion by overwriting ones that have already been used.
• Overwritten Redo Log Files have information that, once overwritten, is lost forever.

ARCHIVELOG Mode:

• If ARCn is in what is termed ARCHIVELOG mode, then as the Redo Log Files fill up, they are individually written to Archived Redo Log Files.
• LGWR does not overwrite a Redo Log File until archiving has completed.
• Committed data is not lost forever and can be recovered in the event of a disk failure.
• Only the contents of the SGA will be lost if an Instance fails.

 NOARCHIVELOG Mode:

• The Redo Log Files are overwritten and not archived.
•  Recovery can only be made to the last full backup of the database files.
• All committed transactions after the last full backup are lost, and you can see that this could cost the firm a lot of $$$.

When running in ARCHIVELOG mode, the DBA is responsible to ensure that the Archived Redo Log Files do not consume all available disk space! Usually after two complete backups are made, any Archived Redo Log Files for prior backups are deleted.