Common Backup Strategies
A good tape rotation schedule is vital to ensure data recovery. The best rotation schedule is one that provides you with a long and varied history of file versions. What follows is a description of two popular rotation schedules. Both provide a great depth of file versions and are offered as configurable backup patterns by most backup application software.
Grandfather-Father-Son Backup Scheme
This is the most commonly used media rotation schedule. This scheme uses daily (Son), weekly (Father), and monthly (Grandfather) backup sets. Four backups tapes are labeled for the day of the week each backs up;
For example, Monday through Thursday. Typically incremental backups are performed on the "Son" group of tapes. These tapes are reused each week on the day matching its label. A set of up to five weekly backup tapes is labeled "Week 1," "Week 2," and so on. Full backups are recorded weekly, on the day that a "Son" media is not use. This "Father" media is reused monthly. The final set of three tapes is labeled "Month 1," "Month 2," and so on, according to which month of the quarter they will be used. This "Grandfather" tape records full backups on the last business day of each month and is reused quarterly. Each of these tapes may be a single tape or a set of tapes, depending on the amount of data to be backed up. A total of 12 tapes are required for this basic rotation scheme, allowing for a history of two to three months.
Because a longer history is often required, archive tapes are periodically pulled from the rotation and replaced with new tapes.
Tower of Hanoi
The Tower of Hanoi rotation schedule is widely used. In this schedule, one tape set "A" is used every other backup session (daily sessions in this example). Start Day 1 with "A" and repeat every other backup session (every other day). The next tape set "B" starts on the first non- "A" backup day and repeats every fourth backup session. Media set "C" starts on the first non-"A" or non- "B" backup day and repeats every eighth session. Media set "D" starts on the first non-"A", non-"B", or non-"C" backup day and repeats every sixteenth session. Media set "E" alternates with media set "D".
With each additional tape set added to the rotation scheme, the backup history doubles. The frequently used tape sets have the most recent copies of a file, while less frequently used media sets have the most recent copies of a file, while less frequently used tape sets have the most recent copies of a file, while less frequently used tapes older versions. This schedule can be used in either a daily or weekly rotation scheme. The decision regarding the frequency of rotation should be based on the volume of data traffic. To maintain the required history of file versions, a minimum of five tape sets should be used in the weekly rotation schedule, or eight for a daily rotation scheme. As with the Grandfather-Father-Son rotation scheme, tapes should be periodically removed from the rotation for archive purposes.
For a more detailed version of the "Tower of Hanoi" media rotation schedule we have provided the following link: www.cut-the-knot.com/recurrance/hanoi.html
