Backup rotation scheme
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A Backup rotation scheme is a method for effectively backing up data where multiple media format are used in the backup process, a common example is the rotation of backup tapes on a regular basis.
As a backup process is trying to achieve a number of objectives different techniques have evolved over time:
Some common rotation schemes are:
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[edit] 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.
[edit] Grandfather, Father, Son
- Keeps a longer archival process
- 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.
Refer specifically to Grandfather-Father-Son Backup.
[edit] Tower of Hanoi
Is a more complex rotation method, its 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 identified with a disk in the puzzle, and every disk movement to a different peg implies a backup to that tape.
[edit] 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, as the system is too complex to calculate the next media mentally.
