Step 5 - Oracle Background Processes
Learn Oracle - Oracle Background Processes
Oracle creates a set of background processes for an instance that interact with each
other and with the operating system to manage memory structure, asynchronously
perform I/O to write data to disk, and do general housekeeping. The background
processes consolidate functions that would otherwise be handled by multiple Oracle
programs running for each user process. They asynchronously perform I/O and
monitor other Oracle processes to provide increased parallelism for better
performance and reliability.
There are many background processes and not all may be present depending upon the
features that are being used in the database. The most common background processes,
and ones that most directly affect Oracle DBA, are the following:
Background Process Description
Database Writer (DBWn)
The database writer writes modified blocks from the database
buffer cache to the files on disk. Oracle allows a maximum of 20
database writer processes.
Log Writer (LGWR)
The log writer process writes redo log entries to disk. Redo log
entries are generated in the redo log buffer of the SGA and the log
writer process writes the redo log entries sequentially into an
online redo log file.
Checkpoint At specific times, all modified database buffers in the SGA are
written to the datafiles by a database writer process (DBWn). This
event is called a checkpoint. The checkpoint process is responsible
for signalling DBWn at checkpoints and updating all of the
datafiles and control files of the database to indicate the most recent
System Monitor (SMON)
The system monitor performs crash recovery when a failed instance
starts up again.
Process Monitor (PMON)
The process monitor performs process recovery when a user
process fails. It is responsible for cleaning up the cache and freeing
resources that the failed process was using.
When the database is running in archive log mode, one or more
archiver processes copy the redo log files to archival storage when
the log files are full or a log switch occurs.