A full backup reads the entire file and copies all blocks into the backup set, only skipping datafile blocks that have never been used.
About Incremental Backups
Rman create backup only changed block since a previous backup. You can use RMAN to create incremental backups of datafiles, tablespaces, or the whole database.
How Incremental Backups Work
Each data block in a datafile contains a system change number (SCN), which is the SCN at which the most recent change was made to the block. During an incremental backup, RMAN reads the SCN of each data block in the input file and compares it to the checkpoint SCN of the parent incremental backup. RMAN reads the entire file every time whether or not the blocks have been used.
The parent backup is the backup that RMAN uses for comparing the SCNs. If the current incremental is a differential backup at level n, then the parent is the most recent incremental of level n or less. If the current incremental is a cumulative backup at level n, then the parent is the most recent incremental of level n-1 or less. 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.
Multilevel Incremental Backups
RMAN can create multilevel incremental backups. Each incremental level is denoted by an integer, for example, 0, 1, 2, and so forth. A level 0 incremental backup, which is the base for subsequent incremental backups, copies all blocks containing data. The only difference between a level 0 backup and a full backup is that a full backup is never included in an incremental strategy.
If no level 0 backup exists when you run a level 1 or higher backup, RMAN makes a level 0 backup automatically to serve as the base.
The benefit of performing multilevel incremental backups is that RMAN does not back up all blocks all of the time.
Differential Incremental Backups
In a differential level n incremental backup, RMAN backs up all blocks that have changed since the most recent backup at level n or lower.
For example, in a differential level 2 backups, RMAN determines which level 2 or level 1 backup occurred most recently and backs up all blocks modified after that backup. If no level 1 is available, RMAN copies all blocks changed since the base level 0 backup. If no level 0 backup is available, RMAN makes a new base level 0 backup for this file.
Case 1: if you want to implement incremental backup strategy as a DBA in your organization:
Use Command for incremental Level Backup
RMAN> backup incremental level 0 database tag="SUNDAY";
RMAN> backup incremental level 3 database tag="MONDAY";
RMAN> backup incremental level 3 database tag="TUESDAY";
RMAN> backup incremental level 3 database tag="WEDNESDAY";
RMAN> backup incremental level 2 database tag="THURSDAY";
RMAN> backup incremental level 3 database tag="FRIDAY";
RMAN> backup incremental level 3 database tag="SATURDAY";
Backup Example ( You can view your incremental Backup Details by using following Query)
select incremental_level, incremental_change#, checkpoint_change#, blocks from v$backup_datafile;
Result of above Query:
INC_LEVEL INC_CHANGE# CHECKPOINT_CHANGE# BLOCKS
0 0 271365 59595
3 271365 271369 2
3 271369 271371 1
3 271371 271374 2
2 271365 271378 2
3 271378 271380 1
3 271380 271383 2
Cumulative Incremental Backups
RMAN provides an option to make cumulative incremental backups at level 1 or greater. In a cumulative level n backup, RMAN backs up all the blocks used since the most recent backup at level n-1 or lower.
For example, in cumulative level 2 backups, RMAN determines which level 1 backup occurred most recently and copies all blocks changed since that backup. If no level 1 backups is available, RMAN copies all blocks changed since the base level 0 backup.
Cumulative incremental backups reduce the work needed for a restore by ensuring that you only need one incremental backup from any particular level. Cumulative backups require more space and time than differential backups, however, because they duplicate the work done by previous backups at the same level.
Case 1: if you want to implement Cumulative backup strategy as a DBA in your organization:
Use Command for Cumulative Level Backup
backup incremental level=0 database tag='base';
backup incremental level=2 cumulative database tag='monday';
backup incremental level=2 cumulative database tag='tuesday';
backup incremental level=2 cumulative database tag='wednesday';
backup incremental level=2 cumulative database tag='thursday';
backup incremental level=2 cumulative database tag='friday';
backup incremental level=2 cumulative database tag='saturday';
backup incremental level=1 cumulative database tag='weekly';
Incremental backup implementation
RMAN will determine the incremental SCN for each datafile
Find the backup with highest checkpoint scn that
- belongs to the incarnation of datafile
- matches the given file#
- is an incremental backup/copy at level N or less if noncumulative or
- is an incremental backup/copy at level N-1 or less if cumulative
- belongs to an available backup set if backup
You can implement a three-level backup scheme so that a full or level 0 backup is taken monthly, a cumulative level 1 backup is taken weekly, and a cumulative level 2 is taken daily. In this scheme, you never have to apply more than a day's worth of redo for complete recovery. When deciding how often to take full or level 0 backups, a good rule of thumb is to take a new level 0 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 when a new level 0 is appropriate. The following query displays the number of blocks written to a backup set for each datafile with at least 50% 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 > .5
ORDER BY COMPLETION_TIME;
Compare the number of blocks in differential or cumulative backups to a base level 0 backup. For example, if you only create level 1 cumulative backups, then when the most recent level 1 backup is about half of the size of the base level 0 backup, take a new level 0.
