Beyond web logs

Thanks to www.span.com for this article which describes backup scheme in very detail & easy way.

There are many backup strategies out there and you have to pick the one that suits you the best. Here are examples of the most used three types:

Grandfather-Father-Son
The most commonly used media rotation schedule is "Grandfather-Father-Son." This scheme uses daily (Son), weekly (Father), and monthly (Grandfather) backup sets.

The GFS scheme begins with the daily backups. Typically, four backup media are labeled for the day of the week each backs up; for example, Monday through Thursday. Each tape is recalled for use on its labeled day. If only a one-week version history of files is maintained, then each tape is overwritten each week. In order to maintain a 3-week version history of files (recommended), more tapes are required. For example this weeks Monday tape will not be overwritten for 3 weeks.

Weekly backups follow a similar scenario. A set of up to five weekly backup media is labeled "Week1," "Week 2," and so on. Full backups are recorded weekly, on the day that a "Son" media is not used. Following the example above these would be "Friday" tapes. This "Father" media is re-used monthly. Five weekly tapes are required in order to maintain a one-month history of files, as some months have 5 weeks.

The final set of three media is labeled "Month1," "Month2," and so on, according to which month of the quarter they will be used. This "Grandfather" media records full backups on the last business day of each month. If your backup plan follows a corporate fiscal calendar, then your monthly tape will take the place of the week 4 or week 5 weekly/Father tape, depending on the month. If your backup schedule follows calendar months, then your monthly backup will vary throughout the year, replacing a daily or weekly tape. Typically, monthly tapes are overwritten quarterly or yearly (recommended), depending on version history requirements.

Each of these "media" may be a single tape or a set of tapes, depending on the amount of data to back up and the type of backup used (incremental vs. full). Weekly and/or monthly tapes are generally pulled as archive tapes.

Tower of Hanoi
In the Tower of Hanoi backup rotation schedule every disc is a backup media set and every move is a day of a backup. In this case, the earlier a backup media set is used, the more often it is used throughout the backup process. Each additional backup set, added to the backup rotation schedule, is used when the previous ones are not used, and doubles the backup history by keeping an older version of data when it is not used.

The Tower of Hanoi rotation schedule allows having a longer backup history, when compared with the Grandfather-father-son rotation schedule. 

In this schedule, one media set "A" is used every other backup session (daily sessions in this example). Start Day 1 with "A" and repeat every other backup (every other day).

The next media 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."

The advantage to the Tower of Hanoi scheme is that with each new media set added to the rotation, the backup history doubles. The frequently used media sets have the most recent copies of a file, while less frequently used media retain older versions.

This backup scheme can be difficult to keep track of manually and therefore is generally done with the help of rotation schemes provided in backup software packages.

As with the Grandfather-Father-Son rotation scheme, tapes should be periodically removed from the rotation for archive purposes.

Incremental Tape Method
This method has a few names and is fairly simple to implement. It involves determining how long you wish to maintain a copy of your data and how many tapes you wish to use. It is based on labeling each tape with a number and then incrementing them through adding and removing one Backup Set each week. It can be configured to allow for either 5-or-7 day backup schemes. An incremental tape rotation method is set up as follows.

The first week you use 1-2-3-4-5-6-7
The second week you use 2-3-4-5-6-7-8
The third week you use 3-4-5-6-7-8-9
The fourth week you use 4-5-6-7-8-9-10
The fifth week you use 5-6-7-8-9-10-11
Tape 1 would then be inserted again 6-7-8-9-10-11-1

You continue this as long as you have tapes and have one tape from every week that you perform a backup able to be stored for a certain period of time. It puts even usage on each tape making sure that a file gets copied to a multiple amount of tapes. The disadvantage is that the backup time can take a while if doing a full backup of multiple servers. It could be varied to do a full backup on the first of every week and then incremental of differential backups every day after that.

An advantage of this system is that tapes can be removed or added to the system at any time if an archive tape or longer file histories are needed. The key is to keep a log of the tape sequence and what date it was last used. This can be calculated months at a time or even for an entire year if necessary.

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