Thursday, 24 March 2016

Oracle RMAN Inremental Backup Algorithm and Change Tracking File

During an incremental backup, RMAN reads the SCN of each data block in the input file and compares it to the checkpoint SCN
of the parent incremental backup. If the SCN in the input data block is greater than or equal to the checkpoint SCN of the
parent, then RMAN copies the block.


Performance of incremental Backup can be improved by enabling block change tracking.When Change tracking is enabled, all the changes to datablocks are recored in change tracking file. RMAN can read the changed SCN's from change tracking file, instead of reading each and every input file.
One change tracking file is created for the entire database.
By default, the change tracking file is created as an Oracle managed file in DB_CREATE_FILE_DEST. We can specify the name of the block change tracking file by providing the location you choose.


SQL command to enable block change tracking

SQL> ALTER DATABASE ENABLE BLOCK CHANGE TRACKING;
This creates change tracking file in "db_create_file_dest" directory


SQL> ALTER DATABASE ENABLE BLOCK CHANGE TRACKING  USING FILE '/test1/data/rman_change_track.f' REUSE;
This creates the change tracking file in the location specified.


SQL command to disable block change tracking

SQL> ALTER DATABASE DISABLE BLOCK CHANGE TRACKING;
By Default,  block change tracking is disabled.
If the change tracking feature is disabled, any existing change tracking files will be deleted


SQL query to find change tracking file Details

select filename, status from v$block_change_tracking


How to Relocate the change tracking file


1.Shut down the database

SHUTDOWN IMMEDIATE

2.Using OS commands, move the change tracking file to a new location.

3.Mount the database and move the change tracking file to a location that has more space.

SQL query to rename change tracking file

ALTER DATABASE RENAME FILE   '/test1/data/rman_change_track.f' TO '/newtest1/data1/change_trk.f';

4.Open the database:

ALTER DATABASE OPEN;


If we cannot afford downtime for database shutdown, change tracking file can be relocated as below


ALTER DATABASE DISABLE BLOCK CHANGE TRACKING;
ALTER DATABASE ENABLE BLOCK CHANGE TRACKING USING FILE 'new-location';

Contents of change tracking file will be lost if we use the above method.So its not a preferable method.
RMAN will have to scan the entire file until the next level 0 incremental backup is done

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