backup rotation

Backup rotation scheme

A backup rotation scheme is a method for effectively backing up data where multiple media (such as tapes) are used in the backup process. The scheme determines how and when each piece of removable storage is used for a backup job and how long it is retained once it has backup data stored on it. Different techniques have evolved over time to balance data retention and restoration needs with the cost of extra data storage media. Such a scheme can be quite complicated if it takes incremental backups, multiple retention periods, and off-site storage into consideration.


Incremental backup

  • Used to keep the longest possible tail of daily backups
  • Archived backups not as important (ie no need to go back 1 year)
  • Useful when data before the rotation period is irrelevant

Basically, the incremental backup is just backing up onto the oldest media in the set. So with a daily backup onto a set of 14 media, you would have 14 days worth of individual daily backups, when all the tapes are used, the oldest one is inserted.

This is the simple method that first comes to mind to most new computer users wanting to do backups. It was commonly used when people backed up regularly to floppy disks.

Grandfather, Father, Son

  • Enables certain backups to be kept much longer.
  • Have more copies available of recent backups, and progressively less as you need to go back over time.

This is one of the most popular method as it achieves multiple aims in having multiple recent backups as well as the ability to refer to past revisions as well as archived data.

Towers of Hanoi

The Towers of Hanoi rotation method is more complex. It is based on the mathematics of the Tower of Hanoi puzzle, with what is essentially a recursive method. It is a 'smart' way of archiving an effective number of backups as well as the ability to go back over time, but it is more complex to understand. Basically, every tape is associated with a disk in the puzzle, and every disk movement to a different peg corresponds with a backup to that tape. So the first tape is used every other day (1, 3, 5, 7, 9,...), the second tape is used every fourth day (2, 6, 10, ...), the third tape is used every eighth day (4, 12, 20, ...).

A set of n tapes (or tapes sets) will allow backups for 2 n - 1 days before the last set is recycled. So, three tapes will give four days worth of backups and on the fifth day Set C will be overwritten; four tapes will give eight days, and Set D is overwritten on the ninth day; five tapes will give 16 days, etc. Files can be restored from 1, 2, 4, 8, 16, ..., 2 n - 1 days ago.

Mathematically, you can look at the sequence of the binary notation of an incrementing n-bit number (starting from zero). In each step, the position of the rightmost zero determines the tape number to use.

The following tables show which tapes are used on which days of various cycles.

Three-Tape Hanoi Schedule

Day of the Cycle
1 2 3 4 5 6 7 8
Set A A A A

Four-Tape Hanoi Schedule

Day of the Cycle
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Set A A A A A A A A

Five-Tape Hanoi Schedule

Day of the Cycle
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
Set A A A A A A A A A A A A A A A A

Incremented media method

This method has many variations and names. A set of numbered media is used until the end of the cycle. Then the cycle is repeated using media numbered the same as the previous cycle, but incremented by one. The lowest numbered tape from the previous cycle is retired and kept permanently. Thus, one has access to every backup for one cycle, and one backup per cycle before that. This method has the advantage of ensuring even media wear, but requires a schedule to be precalculated. The system is generally too complex to mentally calculate the next media to be used.

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