The Backup Strategy Guide

Contents:	1. Introduction to tape backups 2. Tape rotation systems 3. Implementing a backup system

2. Tape Rotation Systems

In a tape rotation system, multiple tapes are organised into a tape backup pool, or tape backup library to provide you with data recovery capabilities, whilst allowing for selected backup tapes to be stored off-site for added security.

Different tapes are used for different days' backups according to a predefined system. Three such tape rotation systems are described here.

Five tape rotation - one per day

The simplest tape rotation scheme is to have one tape for each day of the working week. Tapes are labelled: Monday, Tuesday, Wednesday, Thursday, Friday.

For example, consider the calendar below:

Monday	Tuesday	Wednesday	Thursday	Friday
	1 Tuesday	2 Wednesday	3 Thursday	4 Friday
7 Monday	8 Tuesday	9 Wednesday	10 Thursday	11 Friday
14 Monday	15 Tuesday	16 Wednesday	17 Thursday	18 Friday
21 Monday	22 Tuesday	23 Wednesday	24 Thursday	25 Friday
28 Monday	29 Tuesday	30 Wednesday

You can restore data from any one of the tapes in your library - or in this case, any day in the past week. This strategy requires only five tapes, but only provides one week's data backup history.

Grandfather - Father - Son (GFS)

The grandfather - father - son schedule is the most widely used method, and involves backing up data in the following way:

• daily - on the "son tapes"
• weekly - on the "father tapes"
• monthly - on the "grandfather tapes"

This system is far more powerful than the five tape rotation, but requires more tapes.

For example, consider the calendar below:

Monday	Tuesday	Wednesday	Thursday	Friday
	1 Tuesday	2 Wednesday	3 Thursday	4 Friday
7 Month 1	8 Tuesday	9 Wednesday	10 Thursday	11 Friday
14 Week 2	15 Tuesday	16 Wednesday	17 Thursday	18 Friday
21 Week 3	22 Tuesday	23 Wednesday	24 Thursday	25 Friday
28 Week 4	29 Tuesday	30 Wednesday

This strategy provides you with the ability to restore data from the last week, plus any Monday over the last month, plus any month for as many monthly tapes as you have. Variations on this scheme are available, and provide a trade-off between the number of tapes required, and the number of monthly tapes available.

Tower of Hanoi

The Tower of Hanoi a complex strategy where five tapes are used - called A, B, C, D, E.

• A is used every other day
• B is used every 4th day
• C is used every 8th day
• D and E are used every 16th day, alternating

This ensures that data is available from the last day, 2 days ago, and three other times in history. However, the range of history of backup is dependent on where one is in the cycle.

Consider the following calendar:

Monday	Tuesday	Wednesday	Thursday	Friday
	1 A	2 B	3 A	4 C
7 A	8 B	9 A	10 D	11 A
14 B	15 A	16 E	17 A	18 B
21 A	22 C	23 A	24 B	25 A
28 D	29 A	30 B

This method is clearly confusing, but has the advantage of only requiring 5 tapes. Unless aided by software, this method is not recommended because it is prone to human error.

Next: 3. Implementing a backup system