Cummulative and incremental difference

Posted on 2006-06-08
Last Modified: 2011-04-14
What are the differences between cummulative and differential backups? Please explain briefly.
Question by:expertblr
    LVL 19

    Accepted Solution

    Both are incremental backups. The difference is in the baseline backup used to identify the modified blocks that need to be backed up.

    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.
    LVL 34

    Assisted Solution

    A cummulative backup will be larger, take longer to create (and take more space on disk) but support a faster recovery.  A differential backup will take less space on disk, and will be created faster than a cumulative backup, but it will take longer to recover from, since multiple differential backups may need to be applied.

    An example may help.  Let's say that you do a full backup on Sunday night.  On Monday night, a cumulative or differential backup will be equal, since they will each include all blocks changed since Sunday.  On Tuesday night however they will be different.  A cumulative on Tuesday night will include all blocks changed on either Monday or Tuesday.  An incremental on Tuesday will only include the blocks changed on Tuesday after Monday's incremental was made.

    I you need to recover your database from Tuesday's backup, you need to start with the full Sunday backup in either case.  Then if you have a cumulative from Tuesday, you simply apply that and you are done.  If you have only incrementals however, you first need to apply the Monday incremental, then the Tuesday incremental.

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