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Posted May 12, 2004

Oracle Background Processes - Page 2

By Amar Kumar Padhi

System Monitor

Process Name: SMON

Max Processes: 1

This process is responsible for instance recovery, if necessary, at instance startup. SMON also cleans up temporary segments that are no longer in use. It also coalesces contiguous free extents in dictionary managed tablespaces that have PCTINCREASE set to a non-zero value.

SMON wakes up about every 5 minutes to perform housekeeping activities. SMON must always be running for an instance.

Process Monitor

Process Name: PMON

Max Processes: 1

This process is responsible for performing recovery if a user process fails. It will rollback uncommitted transactions. PMON is also responsible for cleaning up the database buffer cache and freeing resources that were allocated to a process. PMON also registers information about the instance and dispatcher processes with network listener.

PMON wakes up every 3 seconds to perform housekeeping activities. PMON must always be running for an instance.

Checkpoint Process

Process Name: CKPT

Max processes: 1

Checkpoint process signals the synchronization of all database files with the checkpoint information. It ensures data consistency and faster database recovery in case of a crash.

CKPT ensures that all database changes present in the buffer cache at that point are written to the data files, the actual writing is done by the Database Writer process. The datafile headers and the control files are updated with the latest SCN (when the checkpoint occurred), this is done by the log writer process.

The CKPT process is invoked under the following conditions:

1. When a log switch is done.

2. When the time specified by the initialization parameter LOG_CHECKPOINT_TIMEOUT exists between the incremental checkpoint and the tail of the log; this is in seconds.

3. When the number of blocks specified by the initialization parameter LOG_CHECKPOINT_INTERVAL exists between the incremental checkpoint and the tail of the log; these are OS blocks.

4. The number of buffers specified by the initialization parameter FAST_START_IO_TARGET required to perform roll-forward is reached.

5. Oracle 9i onwards, the time specified by the initialization parameter FAST_START_MTTR_TARGET is reached; this is in seconds and specifies the time required for a crash recovery. The parameter FAST_START_MTTR_TARGET replaces LOG_CHECKPOINT_INTERVAL and FAST_START_IO_TARGET, but these parameters can still be used.

6. When the ALTER SYSTEM SWITCH LOGFILE command is issued.

7. When the ALTER SYSTEM CHECKPOINT command is issued.

Incremental Checkpoints initiate the writing of recovery information to datafile headers and controlfiles. Database writer is not signaled to perform buffer cache flushing activity here.

Lock Monitor

Process Name: LMON

processes: 1

Meant for Parallel server setups, Lock Monitor manages global locks and resources. It handles the redistribution of instance locks whenever instances are started or shutdown. Lock Monitor also recovers instance lock information prior to the instance recovery process. Lock Monitor co-ordinates with the Process Monitor to recover dead processes that hold instance locks.

Lock Manager Daemon

Process Name: LMDn

Max Processes: -

Meant for Parallel server setups, LMDn processes manage instance locks that are used to share resources between instances. LMDn processes also handle deadlock detection and remote lock requests.

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