Oracle Database Processes definition and purposes

This the full list of Oracle Database Processes definition and purposes

Process Monitor
PMON
 Responsible for cleaning up after abnormally terminated connections. Responsible for monitoring other server processes and restarting them if necessary Registers the instance with the listener dynamically (dynamic service registration). Restarts failed server processes and dispatcher processes
System Monitor
SMON
 Temporary space cleanup Crash recovery apon restart Coalescing free space Recovering transactions active against unavailable files Instance recovery of failed node in OPS (Oracle parallel server) Cleans up OJB$ (Low Level data dictionary) Shrinks rollback segments Offline's rollback segmentsOther processes call the SMON process when required.
Distributed database recovery
RECO
 Recovers transactions that are left in a prepared state because of a crash or loss of connection during a two-phase commit.
Checkpoint process
CKPT
 The checkpoint process is charged with instructing the database block buffer writers to write the database buffer cache to disk, it then updates the data file headers and control file to indicate when the checkpoint was performed. There is a relationship with checkpoints and recovery time, the more checkpointing the less recovery time is need when a crash occurs. The ckpt process does not do the checkpoint but assists with the checkpointing process by updating the file headers of the data files. A checkpointing process involves the following:
  • Flushing the redo log buffers to the redo log files
  • Writing a checkpoint record to the redo log file
  • Flushing the database log buffers to the data files
  • Updating the data file headers and control files after the checkpoint completes
Database block writer
DBWn
 Responsible for writing dirty blocks to disk when free space within the database buffer cache is low, it writes the dirty blocks from the buffer cache out to the disk. It uses the LRU (Least Recently Used) algorithm which retains data in the memory based on how long it has been since someone asked for that data. The database buffer cache is flushed to disk
  • when the database issues a checkpoint
  • when a server process can't find a clean reusable buffer after checking a threshold number of buffers
  • every 3 seconds
  • users process has searched to long for a free buffer when reading a buffer into the buffer cache
  • Instance is shutdown
  • tablespace is put in backup mode or offline
  • segment is dropped
If you have multiple CPU's then it is advised to run multiple database writers. Use the DB_WRITER_PROCESSES parameter to increase the number of database writers, the instance has to be rebooted.
Log writer
LGWR
 Responsible for flushing to disk the contents of the redo log buffer located in the SGA. Both committed and uncommitted changes are written to the redo log buffer. The redo log buffer is flushed to disk before the data blocks are written to disk. The redo log buffer is flushed to disk
  • every 3 seconds
  • whenever a user commits a transaction
  • when the redo log buffer is a third full or contains 1 Mb of buffered data
  • before the DBWn process writes when a checkpoint occurs
Archive process
ARCn
 Used when the database is in archive-mode, it copies the online redo log file to another location when LGWR fills up, these log files would be used to perform media recovery. There can be a maximum of ten archive processes running ARC0-ARC9. The LOG_ARCHIVE_MAX_PROCESSES parameter determines how many archive processes will be started (default is 1).
Manageability Monitor
MMON
 Collects statistics to help the database manage itself. The MMON process collects the AWR (automatic workload repository) snapshot information which is used by the ADDM (automatic database diagnostic monitor), also MMON issues alerts when database thresholds are exceeded.
Manageability Monitor Light
MMNL
 The process flushes ASH information to disk when the buffer is full, it also captures session history and database metrics.
Memory Manager
MMAN
 Uses the the metrics collected to determine the ideal distribution of memory within oracle. It constantly monitors the database and adjusts the memory allocations according to workloads.
Job Queue Coordination
CJQ0
 Used to schedule and run user jobs. It spawns job queue slave processes (J000-J999) which actually run the job.
Job Queue Process
J000-J999
 These processes are what actually run the schedule jobs requested by CJQ0.
File Mapping Monitor
FMON
 Maps files to immediate storage layers and physical devices. Results are normally kept in the DBMS_STORAGE_MAP view. Generally the 3rd party LVM (logical volume manager) supplier will supply a driver to map to.
Recovery Writer
RVWR
 This process is started when you implement flashback logging, it logs the before image (taken from the flashback buffers) of an oracle block before it is changed, this is written to the flashback log files.
Change Tracking Writer
CTWR
 This process tracks any data blocks that have changed which then RMAN can use to speed up backups as it will no longer need to read the entire data file to see what has changed.
Queue Monitor Coordinator
QMNC
 Spawns and coordinates queue slave processes.
Block server process
BSP
 Used in OPS and keeps each servers SGA in the clusters consistent with each other.
Lock monitor process
LMON
 Used in OPS and monitors all instances in a cluster to detect a failure of an instance.
Lock manager daemon
LMD
 Used in OPS and controls the global locks and global resources for the block buffer cache in a clustered environment.
Lock process
LCKn
 Used in OPS and is the same as the LMD daemon but handles requests for all global resources other than database block buffers
Dispatcher process
Dnnn
 Dispatcher processes that are used when using a shared server environment
Shared Server process
Snnn
 Shared Server processes that are used when using a shared server environment
Oracle process spawner
PSP0
 Process spawner has the job of creating and managing other Oracle processes.
Oracle shadow process
SHAD
 Oracle's shadow process, could not find much on this process
Streams Advanced Queuing process
q000 - q???
I believe this is something to do with Oracle Streams Advanced Queuing

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