What is a Checkpoint?

A Checkpoint is a database event which synchronizes the modified data blocks in memory with the datafiles on disk and CKPT process updates the control file and datafile headers with last SCN.

A checkpoint has two purposes: (1) to establish data consistency, and (2) enable faster database recovery.   How is recovery faster?  Because all database changes up to the checkpoint have been recorded in the datafiles, making it unnecessary to apply redo log entries prior to the checkpoint. The checkpoint must ensure that all the modified buffers in the cache are really written to the corresponding datafiles to avoid the loss of data which may occur with a crash (instance or disk failure).

Oracle writes the dirty buffers to disk only on certain conditions:
  - A shadow process must scan more than one-quarter of the db_block_buffer parameter.
  - Every three seconds.
  - When a checkpoint is produced.

A checkpoint is realized on five types of events:
  - At each switch of the redo log files.
  - When the delay for LOG_CHECKPOINT_TIMEOUT is reached.
  - When the size in bytes corresponding to :
     (LOG_CHECKPOINT_INTERVAL* size of IO OS blocks) is written on the current redo log file.
  -  Directly by the ALTER SYSTEM SWITCH LOGFILE command.
  - Directly with the ALTER SYSTEM CHECKPOINT command.

During a checkpoint the following occurs:
 -  The database writer (DBWR) writes all modified database  blocks in the buffer cache back to datafiles,
 -  Log writer (LGWR) updates both the controlfile and  the datafiles to indicate when the last checkpoint  occurred (SCN)

Various types of checkpoints  in Oracle :

• Full checkpoint
• Thread checkpoint
• File checkpoint
• Parallel Query checkpoint
• Object checkpoint
• Log switch checkpoint
• Incremental checkpoints