Checkpoint

Checkpoint:

A data structure that defines an SCN in the redo thread of a database. Checkpoints are recorded in the control file and each datafile header, and are a crucial element of recovery.

A checkpoint is the highest system change number (SCN) such that all data blocks less than or equal to that SCN are known to be written out to the data files. If a failure occurs, then only the redo records containing changes at SCNs higher than the checkpoint need to be applied during recovery.

A checkpoint performs the following three operations:

  1. Every dirty block in the buffer cache is written to the data files. That is, it synchronizes the datablocks in the buffer cache with the datafiles on disk.
    It’s the DBWR that writes all modified database blocks back to the datafiles.
  2. The latest SCN is written (updated) into the datafile header.
  3. The latest SCN is also written to the controlfiles.

When Checkpoint occurs

1) At redo log switch

2) LOG_CHECKPOINT_TIMEOUT has expired

3) LOG_CHECKPOINT_INTERVAL has reached.

4) Forcefully by DBA ALTER SYSTEM CHECKPOINT

 

Checkpoint Process (CKPT)

A checkpoint process (CKPT) is an Oracle background process that is responsible for recording the details of database checkpoints by making the necessary updates to the headers of the datafiles and the control files.

After every three seconds, the CKPT process records information about the checkpoint position in the online redo log, in the control file. During data recovery process, only redo entries recorded after the checkpoint position are required for data recovery.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: