An Oracle database server consists of an Oracle database and an Oracle instance. An Oracle instance is made up of memory structures, known as the System Global Area (SGA), and background processes that handle much of the behind-the-scenes work involved in running an instance. The most common background processes are the following:
System Monitor (SMON): Performs crash recovery when the instance is started following a failure
Process Monitor (PMON): Performs process cleanup when a user process fails
Database Writer (DBWn): Writes modified blocks from the database buffer cache to the data files on the disk
Checkpoint (CKPT): Updates all the data files and control files of the database to indicate the most recent checkpoint
LogWriter (LGWR): Writes redo log entries to the disk
Archiver (ARCn): Copies redo log files to the archival storage when a log switch occurs
System Monitor (SMON): Performs crash recovery when the instance is started following a failure
Process Monitor (PMON): Performs process cleanup when a user process fails
Database Writer (DBWn): Writes modified blocks from the database buffer cache to the data files on the disk
Checkpoint (CKPT): Updates all the data files and control files of the database to indicate the most recent checkpoint
LogWriter (LGWR): Writes redo log entries to the disk
Archiver (ARCn): Copies redo log files to the archival storage when a log switch occurs
