Oracle Tips by Burleson

Oracle 10g RMAN  - How big is the Block Change Tracking File?

The size of the block change tracking file is proportional to:

• The database size in bytes: the block change tracking file contains data representing every data file block in the database.  The data is approximately 1/250000 of the total size of the database.
   

• The number of enabled threads: In a Real Application Cluster (RAC) environment, the instances update different areas of the tracking file without any locking or inter-node block swapping.  You enable block change tracking for the entire database and not for individual instances.
   

• The number of old backups: The block change tracking file keeps a record of all changes between previous backups, in addition to the modification since the last backup.  It retains the change history for a maximum of eight backups.

The size of the file is calculated by the following formula:

Size of the Block Change Tracking File =

                   ( (Threads*2) + number of old backups ) * database size in bytes
               -------------------------------------------------------------------------
                                                                    250,000

The minimum size for the block change tracking file is 10 MB. Using this formula, a 2 TB database with only one thread and having five backups in the RMAN repository will require a block change tracking file of 59 MB.

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