关于检查点

什么是检查点

 

Checkpoint (CKPT)

At specific times, all modified database buffers in the systemglobal area are written to the datafiles by DBWn. This event is called a checkpoint. The checkpointprocess is
responsible for signalling DBWn at checkpoints and updating all thedatafiles and
control files of the database to indicate the most recentcheckpoint.

 

When a checkpointoccurs, Oracle must update the headers of all datafiles to recordthe details of the checkpoint. This is done by the CKPTprocess. The CKPT process does not write blocks to disk; DBWnalways performs that work.

 

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#Thread checkpoints

The database writes to disk all buffers modified by redo in aspecific thread before a certain target. The set of threadcheckpoints on all instances in a database is a databasecheckpoint. Thread checkpoints occur in the followingsituations:

    *     Consistentdatabase shutdown
    *     ALTER SYSTEMCHECKPOINT statement
    *     Online redolog switch
    *     ALTERDATABASE BEGIN BACKUP statement

#Tablespace and data file checkpoints

The database writes to disk all buffers modified by redo before aspecific target. A tablespace checkpoint is a set of data filecheckpoints, one for each data file in the tablespace. Thesecheckpoints occur in a variety of situations, including making atablespace read-only or taking it offline normal, shrinking a datafile, or executing ALTER TABLESPACE BEGIN BACKUP.

#Incremental checkpoints

An incremental checkpoint is a type of thread checkpoint partlyintended to avoid writing large numbers of blocks at online redolog switches. DBWn checks at least every three seconds to determinewhether it has work to do. When DBWn writes dirty buffers, itadvances the checkpoint position, causing CKPT to write thecheckpoint position to the control file, but notto the data file headers.