增量备份分类
A differential incremental backup, which backs up all blocks changed after the most recent incremental backup at level 1 or 0
A cumulative incremental backup, which backs up all blocks changed after the most recent incremental backup at level 0
Incremental Backups
Cumulative Incremental Backups
Block Change Tracking
The block change tracking 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 stored in the database area. RMAN tracks changed blocks as redo is generated.
If block change tracking is enabled, then RMAN uses the change tracking file to identify changed blocks for incremental backups, 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.
The following concepts are essential for understanding the algorithm that RMAN uses to make incremental backups:
Checkpoint SCN
Every datafile has a datafile checkpoint SCN, which you can view in V$DATAFILE.CHECKPOINT_CHANGE#. All changes with an SCN lower than this SCN are guaranteed to be in the file. When a level 0 incremental backup is restored, the restored datafile contains the checkpoint SCN that it had when the level 0 was created. When a level 1 incremental backup is applied to a file, the checkpoint SCN of the file is advanced to the checkpoint SCN that the file had when the incremental level 1 backup was created.
Incremental start SCN
This SCN applies only to level 1 incremental backups. All blocks whose SCN is greater than or equal to the incremental start SCN are included in the backup. Blocks whose SCN is lower than the incremental start SCN are not included in the backup. The incremental start SCN is most often the checkpoint SCN of the parent of the level 1 backup.
Block SCN
Every data block in a datafile records the SCN at which the most recent change was made to the block.
