Oracle Background processes

To maximize performance and accommodate many users, a multiprocess Oracle system uses some additional Oracle processes called background processes

You can see the Oracle background processes with  following queries
select  * from V$BGPROCESS ;
or
select *  from   v$session where  type ='BACKGROUND';

An Oracle instance can have many background processes; not all are always present

SMON - System Monitor process recovers after instance failure and monitors temporary segments and extents. SMON in a non-failed instance can also perform failed instance recovery for other failed RAC instance.


DBWR - Database Writer or Dirty Buffer Writer process is responsible for writing dirty buffers from the database block cache to the database data files. Generally, DBWR only writes blocks back to the data files on commit, or when the cache is full and space has to be made for more blocks. The possible multiple DBWR processes in RAC must be coordinated through the locking and global cache processes to ensure efficient processing is accomplished.

ARCH - Archive process writes filled redo logs to the archive log location(s). In RAC, the various ARCH processes can be utilized to ensure that copies of the archived redo logs for each instance are available to the other instances in the RAC setup should they be needed for recovery.

CJQ - Job Queue Process (CJQ) - Used for the job scheduler. The job scheduler includes a main program (the coordinator) and slave programs that the coordinator executes. The parameter job_queue_processes controls how many parallel job scheduler jobs can be executed at one time.
CKPT - Checkpoint process writes checkpoint information to control files and data file headers.

CQJ0 - Job queue controller process wakes up periodically and checks the job log. If a job is due, it spawns Jnnnn processes to handle jobs.

FMON - The database communicates with the mapping libraries provided by storage vendors through an external non-Oracle Database process that is spawned by a background process called FMON. FMON is responsible for managing the mapping information. When you specify the FILE_MAPPING initialization parameter for mapping data files to physical devices on a storage subsystem, then the FMON process is spawned.

LGWR - Log Writer process is responsible for writing the log buffers out to the redo logs. In RAC, each RAC instance has its own LGWR process that maintains that instance’s thread of redo logs.

LMON - Lock Manager process

MMON - The Manageability Monitor (MMON) process was introduced in 10g and is associated with the Automatic Workload Repository new features used for automatic problem detection and self-tuning. MMON writes out the required statistics for AWR on a scheduled basis

MMNL - The Memory Monitor Light  process performs frequent and lightweight manageability-related tasks, such as session history capture and metrics computation.

MMAN - The Automatic Shared Memory Management feature uses a new background process named Memory Manager (MMAN). MMAN serves as the SGA Memory Broker and coordinates the sizing of the memory components. The SGA Memory Broker keeps track of the sizes of the components and pending resize  operations

PMON - Process Monitor process recovers failed process resources. If MTS (also called Shared Server Architecture) is being utilized, PMON monitors and restarts any failed dispatcher or server processes. In RAC, PMON’s role as service registration agent is particularly important.

Pnnn - (Optional) Parallel Query Slaves are started and stopped as needed to participate in parallel query operations.

RBAL - This is the ASM related process that performs rebalancing of disk resources controlled by ASM. 

ARBx - These processes are managed by the RBAL process and are used to do the actual rebalancing of ASM controlled disk resources. The number of ARBx processes invoked is directly influenced by the asm_power_limit parameter

ASMB - The ASMB process is used to provide information to and from the Cluster Synchronization Services used by ASM to manage the disk resources. It is also used to update statistics and provide a heartbeat mechanism.

CTWR -  Change Tracking Writer (CTWR) which works with the new block changed tracking features  for fast RMAN incremental backups

WMON - The "wakeup" monitor process

RVWR - Recovery Writer ( RVWR) introduced which is responsible for writing flashback logs which stores pre-image(s) of data blocks

Data Guard/Streams/replication Background processes

DMON - The Data Guard Broker process.


SNP - The snapshot process.

MRP - Managed recovery process - For Data Guard, the background process that applies archived redo log to the standby database.

ORBn - performs the actual rebalance data extent movements in an Automatic Storage Management instance. There can be many of these at a time, called ORB0, ORB1, and so forth.

OSMB - is present in a database instance using an Automatic Storage Management disk group. It communicates with the Automatic Storage Management instance.

RFS - Remote File Server process - In Data Guard, the remote file server process on the standby database receives archived redo logs from the primary database.

QMN - Queue Monitor Process (QMNn) - Used to manage Oracle Streams Advanced Queuing.


Oracle Real Application Clusters (RAC) Background Processes


DIAG: Diagnosability Daemon – Monitors the health of the instance and captures the data for instance process failures.

LCKx - This process manages the global enqueue requests and the cross-instance broadcast. Workload is automatically shared and balanced when there are multiple Global Cache Service Processes (LMSx).

LMON - The Global Enqueue Service Monitor (LMON) monitors the entire cluster to manage the global enqueues and the resources. LMON manages instance and process failures and the associated recovery for the Global Cache Service (GCS) and Global Enqueue Service (GES). In particular, LMON handles the part of recovery associated with global resources. LMON-provided services are also known as cluster group services (CGS)

LMDx - The Global Enqueue Service Daemon (LMD) is the lock agent process that manages enqueue manager service requests for Global Cache Service enqueues to control access to global enqueues and resources. The LMD process also handles deadlock detection and remote enqueue requests. Remote resource requests are the requests originating from another instance.

LMSx - The Global Cache Service Processes (LMSx) are the processes that handle remote Global Cache Service (GCS) messages. Real Application Clusters software provides for up to 10 Global Cache Service Processes. The number of LMSx varies depending on the amount of messaging traffic among nodes in the cluster.
The LMSx handles the acquisition interrupt and blocking interrupt requests from the remote instances for Global Cache Service resources. For cross-instance consistent read requests, the LMSx will create a consistent read version of the block and send it to the requesting instance. The LMSx also controls the flow of messages to remote instances.



The LMSn processes handle the blocking interrupts from the remote instance for the Global Cache Service resources by:
Managing the resource requests and cross-instance call operations for the shared resources.
Building a list of invalid lock elements and validating the lock elements during recovery.
Handling the global lock deadlock detection and Monitoring for the lock conversion timeouts