Database Writer(DBWR):
DBWR writes the dirty buffers from database buffer cache to the datafiles on disk.
It ensures several number of free buffers are available in the database buffer cache.
It will stop writing to the datafiles until one of the following event occurs:
1. CHECK POINT HAPPENS.
2. NO. OF DIRTY BUFFERS REACHES A THRESHOLD VALUE.
3. NO FREE BLOCKS AVAILABLE.
4. PING REQUEST ON RAC ENVI.
5. PUT THE TABLESPACE OFFLINE, READ-ONLY, ALTER TABLESPACE BEGIN BACKUP MODE.
6. DELETE/TRUNCATE A TABLE.
Log Writer
LGWR performs sequential writes from Redo log buffer to the online redo log files.
LGWR writes in the following situations:
1. WHEN A TRANSACTION COMMITS.
2. REDO LOG BUFFER IS 1/3 RD FULL.
3. MORE THAN 1MB OF CHANGES RECORDED IN REDO LOG.
4. BEFORE DBWR WRITES DIRTY BUFFERS.
5. EVERY 3 SECONDS.
SMON-System Monitor:
1. SMON PERFORMS INSTANCE RECOVERY.
2. SPACE MANAGEMENT OF DATAFILES-COELASCING, DE-ALLOCATE TEMPOERARY SEGMENTS
PMON-Process Monitor
CLEANS UP FAILED PROCESSES:
1. ROLLING BACK USER TRANSACTION.
2. RELEASING CURRENTLY HELD TABLE OR ROW LOCKS
3. RESTARTS DEAD DISPATCHERS.
CKPT-Checkpoint
EVERY 3 SEC, CKPT STORES DATA IN CONTROLFILE TO INDICATE AT WHAT POINT THE RECOVERY SHOULD BEGIN IN CASE OF A FAILURE-- CALLED CHECKPOINT.
THE PURPOSE OF THIS IS TO ENSURE ALL THE DIRTY BUFFERS THAT WERE MODIFIED AT A PRIOR POINT IN TIME HAVE BEEN WRITTEN TO THE DATAFILES.
CKPT REDUCES TIME NEEDED FOR RECOVERY-ONLY THE ONLINE REDO LOG ENTRIES FOLLOWING THE LAST CHEPOINT IS NEEDED FOR RECOVERY.
ENSURES ALL THE COMMITED DATA IS WRITTEN TO DATA FILES.
Archiver:(Optional)
1. ARCHIVER PROCESS INITIATES BACKING UP, ARCHIVING OF FILLED ONLINE REDO LOG GROUP AT EVERY LOG SWITCH.
2. MAKES AUTOMATIC ARCHIVING OF FILLED REDO LOG GROUP BEFORE IT CAN BE REUSED.
No comments:
Post a Comment