Incremental Backup Strategy

Choose a backup strategy according to an acceptable MTTR (mean time to recover). For example, you can implement a three-level backup scheme so that a level 0 backup is taken monthly, a cumulative level 1 is taken weekly, and a differential level 1 is taken daily. In this strategy, you never have to apply more than a day of redo for complete recovery.

When deciding how often to take level 0 backups, a good rule of thumb is to take a new level 0 backup whenever 20% or more of the data has changed. If the rate of change to your database is predictable, then you can observe the size of your incremental backups to determine when a new level 0 is appropriate. The following SQL query determines the number of blocks written to an incremental level 1 backup of each datafile with at least 20% of its blocks backed up:

SELECT   FILE#, INCREMENTAL_LEVEL, COMPLETION_TIME,
         BLOCKS, DATAFILE_BLOCKS
FROM     V$BACKUP_DATAFILE
WHERE    INCREMENTAL_LEVEL > 0
AND      BLOCKS / DATAFILE_BLOCKS > .2
ORDER BY COMPLETION_TIME;

Compare the number of blocks in level 1 backups to a level 0 backup. For example, if you only create level 1 cumulative backups, then take a new level 0 backup when the most recent level 1 backup is about half of the size of the level 0 backup.

An effective way to conserve disk space is to make incremental backups to disk, and then offload the backups to tape with BACKUP AS BACKUPSET. Incremental backups are generally smaller than full backups, which limits the space required to store them until they are moved to tape. When the incremental backups on disk are backed up to tape, the tape is more likely to stream because all blocks of the incremental backup are copied to tape. There is no possibility of delay due to time required for RMAN to locate changed blocks in the datafiles.

Another strategy is to use incrementally updated backups as explained in "Incrementally Updating Backups". In this strategy, you create an image copy of each datafile, then periodically roll forward this copy by making and then applying a level 1 incremental backup. In this way you avoid the overhead of making repeated full image copies of your datafiles, but enjoy all of the advantages.

In a Data Guard environment, you can offload incremental backups to a physical standby database. Incremental backups of a standby and primary database are interchangeable. Thus, you can apply an incremental backup of a standby database to a primary database, or apply an incremental backup of a primary database to a standby database.