1、SCN The meaning of ?system change number

     successively 、 Old and new

select dbms_flashback.get_system_change_number,
SCN_TO_TIMESTAMP(dbms_flashback.get_system_change_number) from dual;
SCN Number , To ensure data consistency ;
2、 common SCN
  2.1 Three of the control files SCN
         System SCN

select checkpoint_change# from v$database;

file SCN

select name,checkpoint_change# from v$datafile;

end SCN

select name,last_change# from v$datafile;

Checkpoint information

 Incremental checkpointing does not update the data file header , And the database in the control file SCN And the entry of the data file SCN Information , It's just that every 3 Second by CKPT Process to update... In the control file low cache rba Information , That's the location of the checkpoint .
  2.2 Data file header
    Start SCN

select name,checkpoint_change# from v$datafile_header;
Data block header ITL Transaction slot ( In the following chapters, I will elaborate on )
journal change vector in
    Running log 、 The flight log
In the rollback segment transaction table ( In the following chapters, I will elaborate on )
  2.3 Log file header
-- Check the history log first_change#,next_change#
select recid,sequence#,first_change#,next_change# from v$log_history where rownum<6;
select * from v$log;
select * from v$archived_log;
Under normal circumstances , The system that controls the files SCN file SCN With the header of the data file SCN It should be the same .
The database is closed normally ,buffer cache 
Update system SCN, file SCN, End SCN. The header of the data file SCN.
Abnormal shutdown , End SCN It's empty , Everything else is the same , Abnormal shutdown requires instance recovery . need redo log, No need to file log.
3、 Instance recovery
     Just need redo log:active、current
     Instance recovery judgment basis : When starting the database , Termination of control file detected SCN If it is empty, you need to do instance recovery
     demonstration SCN change

If an instance crash occurs , Just find the checkpoint location in the log file (low cache rba), Apply all redo log files from here , The roll forward operation is completed . After the instance crashes , Start the database again ,oracle It will be read into the control file low cache rba, This is the checkpoint location . Apply redo logs from here , Applied to the on disk rba The location of .on disk rba Is the last redo record in the redo log file on disk rba.

Recover the technical terms involved :
① Running log
    It doesn't matter if you run too much , It won't be applied repeatedly 2 Time , It's just a run away , Don't worry .
② promote SCN, For example, a data file is old , You need to apply archive logs 、 Redo the log to make it SCN Up to the system SCN Agreement .
Every log entry has SCN.
Each log file The head has two SCN,first and next SCN.

In the control file 3 individual SCN:

The header of the data file SCN:


fast_start_mttr_target Parameters ,
The default is 0, Express oracle Automatic adjustment
The unit is seconds .
If it is 20 second , When the instance is restored , Need to roll forward logs ,oracle I'll try to keep it under control 20 Within seconds . Will increase DBWn The frequency of writing .
If the time is too short ,DBWn Write too often ,I/O It's going to be busy , Something could go wrong .

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