What is Checkpoint (CKPT)

The checkpoint process is responsible for updating file headers in the database datafiles. A checkpoint occurs when the Oracle backgroundprocess DBWn writes all the modified database buffers in the SGA including both committed and uncommitted data to the data files. Checkpoints are implemented for the following reasons:

Checkpoints ensure that data blocks in memory that change frequently are written to data files regularly. Because of the least recently used algorithm of DBWn, a data block that changes frequently might never qualify as the least recently used block and thus might never be written to disk if checkpoints did not occur.
Because all database changes up to the checkpoint have been recorded in the data files, redo log entries before the checkpoint no longer need to be applied to the data files if instance recovery is required. Therefore, checkpoints are useful because they can expedite instance recovery.

At a checkpoint, the following information is written:

Checkpoint number into the data file headers
Checkpoint number, log sequence number, archived log names, and system change numbers into the control file.

CKPT does not write data blocks to disk or redo blocks to the online redo logs.Some checkpoints can be logged to the alert log by setting log_checkpoints_to_alert to true.Controlfile and datafile headers are updated CHECKPOINT_CHANGE#

Every 3 seconds CKPT calculates the checkpoint target RBA based on:

The most current RBA

log_checkpoint_timeout

log_checkpoint_interval

fast_start_mttr_target

fast_start_io_target

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