What is a checkpoint?

What is a checkpoint?

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At certain time intervals, the database writer (DBWn) writes all the modified (dirty) buffers from the database buffer cache to the data files on the disk. This event is called a checkpoint. Whenever a checkpoint occurs, the checkpoint process (CKPT) signals the database writer (DBWn) and updates the control file and the headers of data files with the most recent system change number (SCN) to indicate the most recent checkpoint.

A checkpoint automatically occurs whenever a log switch occurs. A log switch occurs when the log writer (LGWR) stops writing to one online redo log group and starts writing to another online redo log group.

A checkpoint also occurs when a database is cleanly shut down (i.e., the database is not shut down using the SHUTDOWN ABORT command), or a tablespace is taken offline using the NORMAL or TEMPORARY option. The database administrator can also force a checkpoint by executing the ALTER SYSTEM CHECKPOINT; statement. When a checkpoint occurs, the data files, online redo log files, and control files are all synchronized.

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