Oracle 9i Database Administration in 10 Minutes

- Asim Abbasi (Author)

Chapter 2: Understanding the Oracle Instance

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Understanding Oracle Instance’s Background Processes

Oracle background processes have been assigned a specific job. Each process gets executed under certain circumstances. If those circumstances happen again, the process gets executed again. Lets look at each of these processes one by one.

DBWn

DBWn or Database Writer process writes the contents of dirty buffers of Database Buffer Cache to the data files.

LGWR

The Log Writer process writes the “Redo Log Buffers” from the Redo Log Cache to the Redo Log files.

ARCn

If we don’t want the Redo Log files to be overwritten by LGWR process then we have to run the database in ARCHIVELOG mode and we have to enable automatic archiving by setting LOG_ARCHIVE_START = True.

Under this mode Archiver process (ARCn) become alive. ARCn process makes sure that the Redo Log files gets archived to the archived location before it gets overwritten by LGWR process. In Oracle 9i, we can have as many as ten Archiver processes (ARC0 to ARC9).

CKPT (Checkpoint)

CKPT process updates the header of data files and control file. It’s a event that happens automatically and kicks the DBWR process to write the dirty buffers to data files.

SMON, PMON

SMON or System Monitor performs instance recovery, if necessary on system startup whereas PMON or Process Monitor frees up all the resources held by a user process whenever it fails.

Dnnn or Dispatcher Process(es)

Dnnn or Dispatcher process is only available if you run Oracle system in Shared Server mode. Its prime job is to take the request from the connected user and place it in the in the Request Queue. Server process when its free picks up the request form the request queue and put the response back in the “Response Queue” after processing the request. Dispatcher process when it’s free picks that response from the response queue and sends it back to the user who submitted that request. You can have multiple Dnnn processes for single Database Instance.

Snnn or Shared Server Process(es)

Shared Server processes (Snnn) utilizing the beauty of Dispatcher process and Request and Response Queue to serve more than one user. The number of Shared Server processes can be controlled using two init.ora parameters, SHARED_SERVERS and MAX_SHARED_SERVERS.

Chapter 2: Understanding the Oracle Instance

Page #: 1 | 2 | 3

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Oracle 9i Database Administration in 10 Minutes