Basics of the Oracle 12c Background Process

In Oracle 12c, you can have over 200 background processes. It says “over 200” because it varies by operating system. If this sounds like a lot, don’t be scared. Many are multiples of the same process (for parallelism and taking advantage of systems with multiple CPUs). Here are the most common background processes.

By default, no processes have more than one instance of their type started. More advanced tuning features involve parallelism. To see a complete list of all the background processes on your OS, query V$BGPROCESS.

Background Process Name Description
PMON The process monitor manages the system’s server processes. It cleans up failed processes by releasing resources and rolling back uncommitted data.
SMON The system monitor is primarily responsible for instance recovery. If the database crashes and redo information must be read and applied, the SMON takes care of it. It also cleans and releases temporary space.
DBWn The database writer’s sole job is taking dirty blocks from the dirty list and writing them to disk. There can be up to 20 of them, hence the n. It starts as DBW0 and continues with DBW1, DBW2, and so on. After DBW9, it continues with DBWa through DBWj. An average system won’t see more than a few of these.
LGWR The log writer process flushes the redo log buffer. It writes the redo entries to disk and signals a completion.
CKPT The checkpoint process is responsible for initiating check points. A check point is when the system periodically dumps all the dirty buffers to disk. Most commonly, this occurs when the database receives a shutdown command. It also updates the data file headers and the control files with the check point information so the SMON know where to start recovery in the event of a system crash.
ARCn Up to 30 archiver processes (0–9, a–t) are responsible for copying filled redo logs to the archived redo storage area. If your database isn’t running in archive mode, this process shuts down.
CJQ0 The job queue coordinator checks for scheduled tasks within the database. These jobs can be set up by the user or can be internal jobs for maintenance. When it finds a job that must be run it spawns the following goodie.
J000 A job queue process slave actually runs the job. There can be up to 1,000 of them (000–999).
DIA0 The diagnosability process resolves deadlock situations and investigates hanging issues.
VKTM The virtual keeper of time sounds like a fantasy game character but simply provides a time reference within the database.
LREG The listener registration process, which registers database instance and dispatcher information with the Oracle listener process. This allows incoming user connections to get from the listener to the database.
MMON The manageablity monitor process supports the Automatic Workload Repository (AWR) by capturing statistics, monitoring threasholds, and taking snapshots. This is related to performance tuning and troubleshooting.
MMNL The manageability monitor lite’s job is to write Active Session History (ASH) statistics from ASH buffer in the SGA to disk. This is related to performance tuning and troubleshooting.

Other background processes exist, as you can tell by the “over 200” number at the beginning. However, those described below are the most common, and you will find them on almost all Oracle installations. When you engage some of Oracle’s more advanced functionality, you’ll see other processes.

It’s very easy to see these background processes if you have an Oracle installation available on Linux or UNIX. The ps –ef |grep ora_ portion lists the background processes. This situation works very well because all background processes begin with ora_.

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