One of the areas that has differentiated backup systems is how tapes are rotated and what files get backed up to which tape. Each rotation method has different advantages that can apply to different systems. Some of the differences between methods are the time it takes to backup and restore, the number of copies and the number of different revisions of a file that get stored on tape. 

When looking into how you are going to develop your backup strategy and rotation method, it is important to look at what data is on your system, how critical it would be if some data were lost, and how fast a system would need to be operational if the system did become damaged or inoperative. You should also assign one individual who is responsible and accountable for administrating the backup to make sure it is done on a consistent basis. Your plan should include a specific time or times when backup is done. For more critical data, you may want to backup files several times during the day while backing up all changed files only once or twice a day. 

The easiest way of backup is where a full image of the system is put on tape every single day. A different tape is cycled through the system each day. This guarantees that a complete restore is able to be done with only one tape. 

The downside is that it takes many tapes, and, if you have a large amount of data, longer backup times each day. This type of backup is common for smaller servers with less than 1 gigabyte of data and that do not need to be up 24 hours a day for operation. You will also need a tape drive that can hold the entire amount of data on the server(s). 

A variation of the above method is where one tape that contains a full image of the system and subsequent tapes receive copies of the files which are different or were updated after the image backup. This type of backup is called Differential backup. This allows the system to be restored with a maximum of just two tapes should a full system restore be needed. 

A variation of this method would be to copy only the files that have changed since the last backup to the tape. This type of backup is called a modified backup, more commonly known as a Incremental backup. This would take less time to backup each time and could be done several times during the day. It would, however, require more time to restore since several tapes may have to be inserted into the tape driver for a total restore. 

Some rotation methods allow for files to be stored multiple times on multiple tapes so that you can have different versions of the same file on multiple tapes. This allows a revision history of files to be stored on the tapes in case a past revision of a file should be needed. This allows a great flexibility in single file restoration, but can hinder the time to restore the system.

• Cost estimating a potential disaster 
• Selecting a Backup Strategy
• Differential Backup's
• Incremental Backup's
• Protecting your Data
• Tower of Hanoi Tape Rotation
• Protecting your investments home page
• Check out our blog at www.daleallen.net where Robert Pajak has written an article about backups.

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