Incremental Backups: RMAN incremental backups back up only data file block that have been changed since last backup. One can make incremental backups at Database level, Tablespace level or datafile level.
The main theme of incremental backup is to back up only those data blocks that have been changed since previous backup.
The reasons for going incremental backup as a part of our backup strategy are:
- To reduce the amount of time needed for taking backups.
- To save network bandwidth when backing up over network.
- For better backup performance.
- To reduce backup size for Non-Archive log databases, instead of going with full database backup one can go with incremental backups.
Incremental Backup Algorithm:
Data is stored in the data blocks. Each data block in a data file contains system change number SCN, which is the SCN at which most recent changes are made to the block. During incremental backup RMAN reads the SCN of each data block in the input file and compares this with the checkpoint SCN of the parent incremental backup. If the SCN in the input data block Is greater than or equal to the checkpoint SCN of the parent, then RMAN copies the block.
Level 0 and Level 1 Backups:
Level 0: Level 0 is just like a full backup which is the base of the subsequent incremental backups, copies all blocks containing data, backing the data file up into a backup set.
The main difference between the full backup and incremental level 0 backups is , full backup is never included in the incremental backup strategy.
Level 1: With reference to the previous level 0 backup or level 1 backup it backs up the modified data blocks.
Types of Level 1 incremental backups:
Differential Backup: Backus up all blocks changed after the most recent incremental backup at level 1 or 0
Cumulative backup: Backups up all blocks changed after the most recent incremental backup at level 0
By default incremental backups are differential.
RMAN> Backup Incremental level 1 Database; (example of differential backup)
RMAN> Backup Incremental level 1 cumulative database; (blocks changed since level 0- Cumulative Incremental Backups.
Incremental Backup Strategy:
Choose a backup schema according to an acceptable MTTR (mean time to recover). For example, you can implement a three ñlevel backup schema so that a full or level 0 backup is taken monthly, a cumulative level 1 is taken weekly, and a differential level 1 is taken daily.
How often we can take full or incremental backups?
When deciding how often we can take full or incremental backups, a good rule of thumb is to take a new level 0 backup whenever 50% 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 new level 0 is appropriate.
The following query will help you to determine the rate of changed blocks.
SELECT FILE#, INCREMENTAL_LEVEL, COMPLETION_TIME, BLOCKS, DATAFILE_BLOCKS
FROM V$BACKUP_DATAFILE
WHERE INCREMENTAL_LEVEL > 0
AND BLOCKS / DATAFILE_BLOCKS > .5
ORDER BY COMPLETION_TIME;
Making Incremental Backups : Backup Incremental
After starting RMAN, run the BACKUP INCREMENTAL command at the RMAN prompt. This example makes a level 0 incremental backup of the database:
BACKUP INCREMENTAL LEVEL 0 DATABASE;
This example makes a differential level 1 backup of the SYSTEM tablespace and datafile tools01.dbf. It will only back up those data blocks changed since the most recent level 1 or level 0 backup:
BACKUP INCREMENTAL LEVEL 1
TABLESPACE SYSTEM
DATAFILE ‘ora_home/oradata/trgt/tools01.dbf’;
This example makes a cumulative level 1 backup of the tablespace users, backing up all blocks changed since the most recent level 0 backup.
BACKUP INCREMENTAL LEVEL = 1 CUMULATIVE
TABLESPACE users;
