The Block Change Tracking File

When using Oracle block change tracking we see this procedure.  As data blocks change, the Change Tracking Writer (CTWR) background process tracks the changed blocks in a private area of memory.

When a commit is issued against the data block, the block change tracking information is copied to a shared area in Large Pool called the CTWR buffer. During the checkpoint, the CTWR process writes the information from the CTWR RAM buffer to the change-tracking file.

The block change tracking file is maintained in the directory defined by the db_create_file_dest parameter, if it is configured. You can also define the location of the block change tracking file via the using file clause of the alter database enable block change tracking command. You can rename the block change tracking file with the alter database rename file command, just as you would with any normal database file. Here is an example of turning on block change tracking, using the using file clause to define where the block change tracking file should be:

Alter database enable block change tacking
using file '/u01/oracle/RMAN/blocktrack/my_db_tracking.chg';
The block change tracking file size depends on svereal considerations, including the number of enabled redo threads, the size of the database, and the number of RMAN backups for which change data needs to be stored. The formula looks like this:
R=((T * 2)+ B) * (S/250000)
where:
R = The size of the block change tracking file, in bytes

T = The number of enabled redo threads

S = The size of the database, in bytes

B = The number of incremental backups for which RMAN needs to store change tracking data

• 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 [sic]. 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.