DBA

Background Processes :-

* The relationship between physical (database) and memory sturture maintain and enforced by the Oracle's Background processes.

* Mandatory backgroun processes,

• DBWR -- Database Writer
• LGWR -- Log writer
• PMON -- Process Monitor
• SMON -- System Monitor
• CKPT -- Checkpoint

Database Writer :-

* Database Writer writes when,

• Checkpoint Occur
• Dirty buffer thershold reached
• No free Buffers
• Timeout
• RAC ping request
• Tablespace offline
• Tablespace read only
• Table DROP or Truncate
• Tablespace BEGIN BACKUP

*db writer writes dirty buffer from the database buffer cache to the datafiles.

What is Dirty Buffer?

* A buffer that is currently not in use, and contains data that has been committed but not written back to the disk by the buffer writer process is called a dirty buffer.

* If a user process finds a dirty buffer in the least recently used (LRU) list, it moves the dirty buffer to the write list and continues searching for more dirty buffers.

Log Writer :-

• At commit
• When one-third full
• When there is 1MB of redo
• Every 3 seconds
• Befor DBWN writes

System Monitor :-

* Instance recovery

• Rolls forward changes in the redo logs.
• Opens the database for user access
• Roll back uncommitted transactions.

* Coalesces free space ever 3 sec

* Deallocates temporary segments.

Process Monitor :-

* Cleans up after failed processes by

• Rolling back the transaction
• Releasing locks
• Releasing Other Resources
• Restarts dead dispatchers

What is Checkpoint?

* A checkpoint occurs when the db writer process writes all modified buffers in the sga buffer cache to the database datafiles.

* Data file headers are also updated with the latest checkpoint SCN, even if the file had no changed blocks.

* Checkpoints occur AFTER (not during) every redo log switch.

* set parameter log_checkpoints_to_alert=true to observe checkpoint start and end times in the database alert log.

* Checkpoints can be forced with the Alter system checkpoint command.

What is SCN?

* System Change Number ---> A number,internal to Oracle that is incremented Over time as change vectors are generated,applied and written to the redo log.

* System commit Number ---> A number,internal to oracle that is incremented with each database commit.