Database Reference
In-Depth Information
Redo Log Buffer
The redo log buffer keeps a log of changes to the data in the memory
area. The size of the redo log is static.
Background Processes
When an Oracle instance is started a set of Oracle processes are started in memory and run
in the background. Background processes are like jobs that perform common tasks to man-
age the proper functioning of the Oracle instance. Several kinds of background jobs are star-
ted at the instance start and perform a specific job function. Some of the examples of back-
ground processes are process monitor, system monitor, and lock process.
Process Monitor (PMON)
For every end-user process there is a 1:1 Oracle shadow pro-
cess. PMON monitors the Oracle shadow process. If a user process crashes, PMON cleans
up the orphaned Oracle shadow process and makes sure the data consistency is maintained.
System Monitor (SMON)
SMON performs recovery functions at instance start, writing an
alert log when an instance process fails and conducting cleanup of temporary segments
when not required.
Lock Process
This background process works as a lock manager monitor.
Recoverer (RECO)
Recoverer manages the in-doubt distributed transactions in distributed
databases.
Other Processes
There are other critical background processes that operate in an Oracle database. The most
important of these are covered in the following sections.
Database Writer (DBWR)
The DBWR writes the data from the database buffer cache to
the data files.
Log Writer (LGWR)
The LGWR writes the redo log buffer to redo log files on the disk.
Archiver (ARC0)
The archiver process will automatically write the online redo logs to
archive log files at an offline storage location (initially, this is the local disk). This process
does this when the Oracle Database is configured to run with archive mode on.
Checkpoint (CKPT)
The CKPT process writes all modified database cache buffers in SGA
to the datafiles.
The following command at the operating system level shows the Oracle background pro-
cesses:
