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SCN (system change number) system change number

Redo block address of RBA (redo block address) onlin redo log

Current SCN of the select dbms_flashback.get_system_change_number from dual; system

The update of SCN is triggered by commit, and the SCN is refreshed every 3 seconds.

The CKPT process updates the current SCN of the database into the database file header and the control file when checkpoint occurs, and the DBWn process writes the dirty data blocks (dirty block) in the buffer cache to the data file, but ckpt notifies the DBWn process after updating the control file and the data file header to generate a checkpoint. The DBWn process will write the dirty data in the buffer cache (the dirty data corresponding to the current online redo log) into our data file, which is basically such a circular process. The ckpt process will also record the RBA in the control file. This address is used to mark the location in the log where a checkpoint corresponds to a rba when the instance is restored. When instance recovery (instance recovery), oracle will know from that checkpoint to start media recovery at that rba in redo!

Condition for creating checkpoints (checkpoint): 1.alter system checkpoint

2.alter system switch logfile

The 3.DBWn process writes out dirty blocks

View the scn in the control file: after executing checkpoint, Oracle updates the System checkpoint SCN in the current control file (stored in the control file)

Select checkpoint_change# from v$database

View the scn in the data file information recorded in the control file: (stored in the control file) the control file records the location and information of each database file of Oracle, including Datafile checkpoint SCN, of course, so when executing checkpoint, Oracle will also update the datafile checkpoint SCN of each data file recorded in the control file

Select checkpoint_change# from v$datafile

Check the start scn in the data file header: (stored in the data file header) when checkpoint is executed, Oracle will update the Start SCN stored in each actual data file header (note that it will definitely not be in the control file). The purpose of this SCN is to check whether media recovery (media recovery) is needed during database startup.

Select checkpoint_change# from v$datafile_header

Check the end scn of the data file in the control file: (stored in the control file) each recorded data file header has a corresponding End SCN, and this End SCN must exist in the control file. The absolute significance of this SCN is mainly used to verify whether instance recovery (instance recovery) is needed during database startup.

Select name,last_change# from v$datafile

For the relationship between SCN and recovery, please refer to:

Https://blog.csdn.net/dba_waterbin/article/details/7758641

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