Block Change Tracking

RMAN Incremental Backups backup only the blocks that were changed since the lastest base incremental backups. But RMAN had to scan the whole database to find the changed blocks. Hence the incremental backups read the whole database and writes only the changed blocks. Thus the incremental backups saves space but the reduction in the time is fairly neglegible.


Block Change Tracking (BCT) is a new feature in Oracle 10g. BCT enables RMAN to read only the blocks that were changed since the lastest base incremental backups. Hence by enabling BCT, RMAN reads only the changed blocks and writes only the changed blocks.


Without BCT, RMAN has to read every block in the database and compare the SCN in the block with the SCN in the base backup. If the block's SCN is greater than the SCN in the base backup then the block is a candidate for the new incremental backup. Usually only few blocks are changed between backups and the RMAN has to do unncessary work of reading the whole database.


BCT Stores the information about the blocks being changed inthe BlockChange Tracking File. The background process that does this logging is Change Tracking Writer (CWTR).


BlockChange Tracking File


BCT File is one per database and in the case RAC, it is shared among all the instances. BCT File is created in the location defined by the parameter DB_CREATE_FILE_DEST as OMF file.


To enable BCT


SQL> Alter Database Enable Block Change Tracking;


To disable BCT


SQL> Alter Database Disable Block Change Tracking;


To specify the BCT file location


SQL> Alter Database enable Block Change Tracking using File '/Backup/BCT/bct.ora';


A useful query,

SQL> Select Completion_time, datafile_blocks, blocks_read, blocks, used_change_tracking
From v$backup_datafile
where to_char(completion_time, 'dd/mon/yy') = to_char(sysdate, 'dd/mon/yy');

Where,
datafile_blocks is the total number of blocks in the datafile.
blocks_read is the total number of blocks read by RMAN
blocks is the total number of blocks backed up by the RMAN.
used_change_tracking if yes BCT is used, if no BCT is not used.

Thanks