How to choose a Backup Rotation Schedule
In the dynamic, rapidly changing world, it is highly important to be confident in one's data security. Regular data backup can give one the desired security. However, such simple move as backup is often neglected, because one never considers such a necessity until the disaster strikes. So what can be done to avoid the disaster or reduce the expenses to the minimum?
The first backup strategy that might come to one's mind is to backup data daily to the same media. But such backup strategy does not provide the required security, since the data can be damaged long before the last backup, nor it is convenient, as performing full backup daily takes a lot of time and wears out the hardware and the backup media. As always, the most simple and the easiest solution is not the best one.
So, basically, the best backup data strategy, including a backup rotation schedule, should be both effective and efficient. It means it will still keep one's data safe and will deal with a reasonable amount of the backup media. The backup rotation schedule would include a combination of full and incremental backups performed at regular intervals. Also it would include a number of media sets, as backup data to a single one is not reliable.
For example, a full backup can be performed once a week and incremental or differential backups can be performed throughout the week from Monday to Thursday.
Nowadays, there are two widely known backup rotation strategies, Grandfather-father-son (also known as GFS or Grandparent-parent-child) and the Tower of Hanoi.
Grandfather-Father-Son
Grandfather-father-son is the most widely used backup rotation schedule. According to this backup rotation schedule incremental backups are performed daily, while full backups are done weekly and monthly.
First of all, from one to four media sets are designated for incremental daily backups. As daily incremental backup backs up the least of the data, compared with other backup sessions in this backup rotation schedule, any of these media sets can be referred to as 'son'. These media sets are reused each week on the designated day.
Secondly, full backups are performed weekly - daily incremental backup is not performed on this day. As the first backup needs to be a full backup, the scheme is started on Friday, the day of the full backup. Consequently, the media set used on Friday can be referred to as 'father'.
Furthermore, on the last business day of the month a full backup is performed to a new media set, which can be referred to as 'grandfather'. These media sets are designated for specific months troughout the year.
As the backup data size can vary, each of these media sets may be a single backup medium or multiple backup media sets. To complete a quarterly backup rotation, a total of twelve backup media sets are required when using this basic backup rotation schedule. Sometimes one may wish backup history needs to be saved - in this case a media set with a full backup is taken from the rotation and replaced with a new media set.
Tower of Hanoi
The Tower of Hanoi backup rotation schedule stems from a puzzle, which was invented by Edouard Lucas, a French mathematician, in 1883. The game consisted of three pegs with eight discs placed on one of them. The task was to move all the discs to another peg, never placing a smaller disc under a larger one.
Just like in the game, the backup media sets are rotated through the entire backup process. Compared with the Grandfather-father-son rotation schedule, this one can employ more backup media sets and save a deeper backup history - the less often the backup media set is used, the older data version it has. This backup rotation schedule requires only five backup media sets to perform backups during a month every day, for instance.
How the Tower of Hanoi backup rotation schedule works
In this backup rotation schedule the first media set is used every second day, the second media set is used when the first set is not used and is used every fourth day, the third media set is used when neither of the previous sets are used and every eighth day and so on.
Each additional backup media set, added to the rotation, doubles the backup history, and is used only when the previous sets are not used, thus, the later a backup media set is added to the history, the less it is used and the older files it stores are. This enables one to have a deep backup history, but does not allow storing many recent backup copies, as a daily backup is erased in two days, for example.
