Block Change Tracking(BCT)

The block change tracking (BCT) feature for incremental backups improves incremental backup performance by recording changed blocks in each datafile in a block change tracking file. This file is a small binary file called block change tracking (BCT) file stored
in the database area. RMAN tracks changed blocks as redo is generated.If we enable block change tracking, then RMAN uses the change tracking file(BCT)  to identify changed blocks for an incremental backup, thus avoiding the need to scan every block in the
datafile. RMAN only uses block change tracking when the incremental level is greater than 0 because a level 0 incremental backup includes all blocks. 

Enable block change tracking (BCT) 

SQL> alter database enable block change tracking  using file '/archivelog/bct.dbf' ;

When data blocks change, shadow processes track the changed blocks in a private area of memory at the same time they generate redo . When a commit is issued, the BCT information is copied to a shared area in Large Pool called 'CTWR dba buffer' . At the checkpoint,
a new background process, Change Tracking Writer (CTWR) , writes the information from the buffer to the change-tracking file . If contention for space in the CTWR dba buffer occurs, a wait event called , 'Block Change Tracking Buffer Space'  is recorded. Several
causes for this wait event are poor I/O performance on the disk where the change-tracking file resides , or the CTWR dba buffer is too small to record the number of concurrent block changes .By default, the CTWR process is disabled because it can introduce
some minimal performance overhead on the database. 

The v$block_change_tracking  views contains the name and size of the block change tracking file plus the status of change tracking: We can check by the below command :  

SQL> select filename, status, bytes from v$block_change_tracking;

To check whether the block change tracking file is being used or not, use the below command .

SQL> select  file#,  avg(datafile_blocks), avg(blocks_read),  avg(blocks_read/datafile_blocks) * 100 as  "% read for backup"  from v$backup_datafile  where incremental_level > 0  and  used_change_tracking = 'YES'  group by file#   order by file# ;

To disable Block Change Tracking (BCT)   issue the below command : 

SQL> alter database disable block change tracking  ;

Estimating Size of the Change Tracking File on Disk
The size of the change tracking file is proportional to the size of the database and the number of enabled threads of redo. The size is not related to the frequency of updates to the database. Typically, the space required for block change tracking is approximately
1/30,000 the size of the data blocks to be tracked.
Note, however, the following two factors that may cause the file to be larger than this estimate suggests:

To avoid overhead of allocating space as your database grows, the change tracking file size starts at 10MB, and new space is allocated in 10MB incremenents. Thus, for any database up to approximately 300GB the file size is no smaller than 10MB, for up to
approximately 600GB the file size is no smaller than 20MB, and so on.

For each datafile, a minimum of 320K of space is allocated in the change tracking file, regardless of the size of the file. Thus, if you have a large number of relatively small datafiles, the change tracking file is larger than for databases with a smaller
number of larger datafiles containing the same data.