RelationalDBDesign RelationalDBDesign


Instance Architecture   «Prev  Next»
Lesson 15 The Archiver (ARC0)
Objective Explain the function of the Archiver.

Archiver (ARC0)

Storing and copying redo log files

The Achiver process makes a copy of each redo log file as it is filled, and stores that copy in an offline location. It is possible to run an Oracle database without archiving the redo logs, but archiving enables up to the minute recovery from a failure, so most production databases take advantage of it.
The following Slide Show demonstrates what the Archiver process does:

  1. As changes are made to an Oracle database, a log of those changes is written to the redo log files.
  2. As each redo log is filled, the archiver process will begin to copy it to the archive.
  3. The archiver will more or less keep up with the log writer.
  4. If the log writer gets far enough ahead to fill up all the redo log files, then database users will be forced to wait while the archiver copies another file.
  5. When a database is running in archive log mode, then log writer will not overwrite a log file until it has been copied by the archiver.
  6. Once a log file has been copied, processing will resume.

Archiver Functions
Like the Database Writer, you can have up to ten archiver processes running for a single instance in Oracle.
Archiving is the operation of generating an archived redo log file. Archiving is either automatic or manual and is only possible when the database is in ARCHIVELOG mode. An archived redo log file includes the redo entries and the log sequence number of the identical member of the online redo log group. The archived redo log contains a copy of every group created since you enabled archiving.

ARCHIVELOG mode

When Oracle is run in ARCHIVELOG mode, the ARC0-ARCt (Archiver) background processes make a copy of each redo log file before overwriting it. These archived redo log files are usually written to a disk device. The archived redo log files may also be written directly to a tape device, but this tends to be very operator-intensive. You can perform file system backups of a database while that database is open, provided the database is running in ARCHIVELOG mode. An online backup involves setting each tablespace (or the entire database) into a backup state, backing up the related datafiles, and then restoring the tablespaces to their normal state.