Fast Incremental Backups

There are performance issues associated with incremental backups as the whole of each datafile must be scanned to identify changed blocks. In Oracle 10g it is possible to track changed blocks using a change tracking file. Enabling change tracking does produce a small overhead, but it greatly improves the performance of incremental backups. The current change tracking status can be displayed using the following query.

SELECT status FROM v$block_change_tracking;

Change tracking is enabled using the ALTER DATABASE command.

ALTER DATABASE ENABLE BLOCK CHANGE TRACKING;

By default the change tracking file is created as an Oracle Managed File (OMF) in the location pointed to by the DB_CREATE_FILE_DEST parameter. An alternate location can be specified using the following command.

ALTER DATABASE ENABLE BLOCK CHANGE TRACKING 
  USING FILE '/u01/oradata/MYSID/rman_change_track.f' REUSE;

The tracking file is created with a minumum size of 10M and grows in 10M increments. It's size is typically 1/30,000 the size of the datablocks to be tracked.

Change tracking can be disabled using the following command.

ALTER DATABASE DISABLE BLOCK CHANGE TRACKING;

Renaming or moving a tracking file can be accomplished in the normal way using the ALTER DATABASE RENAME FILE command. If the instance cannot be restarted you can simply disable and re-enable change tracking to create a new file. This method does result in the loss of any current change information.