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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