Oracle CheckPoint Usage

A checkpoint is the writing by the DBWR (database writer), process of all modified buffers in the SGA buffer cache To the database data files. Data file headers is also updated with the latest checkpoint SCN, even if the file had no changed blocks, as well as the Control files.

Checkpoints occur after (not during) every redo log switch and also at intervals specified by initialization parameters.

Set parameter log_checkpoints_to_alert=true to observe checkpoint start and end times in the database ALERT LOG.

Checkpoints can forced with the ALTER SYSTEM CHECKPOINT; command.

a number of dirty database buffers covered by the log being checkpointed is written to the data files by DBWN.  The number of buffers being written by DBWN are determined by the Fast_start_mttr_target parameter, if specified. (Oracle official doc).

**asktom***: A checkpoint is the act of Flushing modified, cached database blocks to disk. Normally, when do a change to a block – the modifications of that block is made to a memory copy of the block. When you commit--the block was not written (and the REDO LOG is--that's makes it so we can "replay" your transaction in T He event of a failure). Eventually, the system would checkpoint your modified blocks to disk. There is no relationship between "checkpoint" and SIDS and instance recovery does not imply "checkpoint". A checkpoint reduces the amount of time it takes to perform instance recovery ....

	Oracle Checkpoint
		A checkpoint performs the following three operations: Every dirty block in the buffer cache was written to the data files. That's, it synchronizes the datablocks in the buffer cache with the datafiles on disk. It's the DBWR that writes all modified databaseblocks back to the datafiles. The latest SCN is a written (updated) into the datafile header. The latest SCN is also written to the Controlfiles. The update of the datafile headers and the control files is do by the LGWR (CKPT if CKPT is enabled). As of version 8.0, CKPT is enabled by default. Events that trigger a checkpointThe following events trigger a checkpoint. Redo Log Switch Log_checkpoint_timeout has expired Log_checkpoint_interval has been reached DBA requires SO (alter system checkpoint) Additionally, if a tablespace is hot backuped, a checkpoint for the tablespace in question are taking place.while redo log switches cause a checkpoint, checkpoints don ' t cause a log switch.Time and SCN of last checkpointThe date and time of the last checkpoint can is retrieved through checkpoint_time in V$datafile_headerThe SCN of the last checkpoint can is found in v$database. checkpoint_change#. Size of redo logIf the size of the redo log is to small, the performance of the checkpoint won't be optimal. The If the Alert.log contains messages like Thread. Cannot allocate new log ....

Oracle CheckPoint Usage