The database writer writes modified blocks from the database buffer cache to the files on a disk. Oracle Database allows a maximum of 36 database writer processes.

The log writer process writes redo log entries to disk. Redo log entries are generated in the redo log buffer of the System Global Area (SGA) and the log writer process writes the redo log entries sequentially into an online redo log file.

At specific times, all modified database buffers in the SGA are written to the data files by a database writer process (DBWn). This event is called a checkpoint. The checkpoint process signals DBWn, updates the data files and control files of the database, and records the time of this update.

The system monitor performs instance recovery when a failed instance is restarted.

The process monitor performs a recovery when a user process fails. It cleans up the cache and frees resources that the failed process was using.

Archiver processes copy the online redo log files to archival storage when the log files are full or a log switch occurs. The database must be in archive log mode to run archive processes. For more information, see Chapter 9, "Performing Backup and Recovery".

This process performs various management-related background tasks, for example:

• Issuing alerts whenever a given metric violates its threshold value

• Taking snapshots by spawning additional processes

• Capturing statistical values for SQL objects that have been recently modified

Job queue processes run user jobs, often in batch mode. A job is a user-defined task scheduled to run one or more times.