Oracle Background Processes

May 12, 2004

An Oracle instance runs two types of processes - Server and Background. Server processes are created to handle requests from sessions connected to the instance. Background processes, as the name says, are processes running behind the scene and are meant to perform certain maintenance activities or to deal with abnormal conditions arising in the lifetime of the instance. We will discuss Background processes in detail here.

Not all background processes are mandatory for an instance. Each background process is meant for a specific purpose and its role is well defined. Background processes are visible as separate operating system processes in Unix/Linux. In Windows, these run as separate threads within the same process.

Background processes are invoked automatically when the instance is started. Given below is a list of twenty background processes along with their technical or internally referred names.

Database Writer

Process Name: DBW0 through DBW9 and DBWa through DBWj

Max Processes: 20

This process writes the dirty buffers present in the database buffer cache to datafiles. The process does multi-block writing to disk asynchronously. One database writer process is adequate for most systems; more can be configured if necessary. The initialization parameter, DB_WRITER_PROCESSES, specifies the number of database writer processes to start.

The DBWn process writes dirty buffer to disk under the following conditions:

1. When a checkpoint is issued. Please see checkpoint process below.

2. When a server process cannot find a clean reusable buffer after scanning a threshold number of buffers.

3. Every 3 seconds. See Followup on DBWn DBWn 3 second write

Database writer must always be running for an instance.

Log Writer

Process Name: LGWR

Max Processes: 1

The log writer process writes data from the redo log buffers to the redo log files on disk.

The writer is activated under the following conditions:

1. When a transaction is committed, a System Change Number (SCN) is generated and tagged to it. Log writer puts a commit record in the redo log buffer and writes it to disk immediately along with the transaction's redo entries. Changes to actual data blocks are deferred until a convenient time (Fast-Commit Mechanism).

2. Every 3 seconds.

3. When the redo log buffer is 1/3 full.

4. When DBWn signals the writing of redo records to disk. All redo records associated with changes in the block buffers must be written to disk first (The write-ahead protocol). While writing dirty buffers, if the DBWn process finds that some redo information has not been written, it signals the LGWR to write the information and waits until the control is returned.

Log writer will write synchronously to the redo log groups in a circular fashion. If any damage is identified with a redo log file, the log writer will log an error in the LGWR trace file and the system Alert Log. Sometimes, when additional redo log buffer space is required, the LGWR will even write uncommitted redo log entries to release the held buffers. LGWR can also use group commits (multiple committed transaction's redo entries taken together) to write to redo logs when a database is undergoing heavy write operations.

The log writer must always be running for an instance.

Archiver

Process Name: ARC0 through ARC9

Max Processes: 10

The ARCn process is responsible for writing the online redo log files to the mentioned archive log destination after a log switch has occurred. ARCn is present only if the database is running in archivelog mode and automatic archiving is enabled. The log writer process is responsible for starting multiple ARCn processes when the workload increases. Unless ARCn completes the copying of a redo log file, it is not released to log writer for overwriting.

The number of Archiver processes that can be invoked initially is specified by the initialization parameter LOG_ARCHIVE_MAX_PROCESSES. The actual number of Archiver processes in use may vary based on the workload.








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