Merlin's
Blue*
Fact
Sheets

Computer Backups

   
 

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Some hints to help you develop and implement an effective,
simple and useful backup strategy.

 

What would you do if your computer and all the data in it disappeared overnight? - It could be stolen by a burglar, or the hard disk drive could crash, or water or smoke damage could destroy it. It is very wise to take backup copies of your information from time to time.

What is involved? - Doing a "backup" is the process of duplicating the disk files/information onto either floppy disks, CD or magnetic tape (or some other removeable media). This is in case of inadvertent loss or destruction of the original. The resultant duplicate copy is known as a backup copy.

There are various backup strategies, depending on the specific user requirements, including consideration of the storage of a recent backup copy off-site in case of catastrophic destruction of the computer and the local backup storage facility. In case of a problem with some or all information on the computer system, the backup copy can be re-applied to the system in what is referred to as a "restore: process, to restore the computer system to a sound state as at a previous point in time.

Partial backup versus Complete backup - Depending on the amount of information to be backed up, and the capacity of the backup device, the backup process can either be of the complete computer system, or of just a particular part (eg. just user data and files), or even a selective portion based on criteria such as the latest change date.

Backup Frequency - Daily, Weekly or Monthly - Computer files that potentially change at least daily should be backed up on a daily basis. Files that change less frequently are typically backed up on a weekly, or even monthly basis. eg. It can be good practice to take a complete backup of the entire system on a monthly basis, and to take partial backups on a daily basis.

Backup Rotation - The "Grandfather-Father-Son" backup strategy is where three sets of backup tapes (or disks or other media) are used, and are recycled in sequence. ie. For a daily backup routine, the backup performed today is the first generation (grandfather), tomorrow's backup will be the second generation (father), and the following day's will be the third generation (son). On the following day, the first generation media (grandfather) can be reused.

Incremental backup - This is the set of backup processes where a complete backup is taken of a system on a particular day. The next backup taken (eg. the following day) will be only of the files that have changed since the last complete backup. The next one will likewise be only of changed files, and so on for a set number of backups (eg. for a week). This then becomes one set of backups. To restore from this set of backups, it can be necessary to search the first complete backup for the required file(s), and then search each intermediate incremental backup until they are found.

Also see: Contingency Planning and Disaster Recovery

* - Merlin's Blue Fact Sheets are so named because the first versions (produced in 1989) were printed onto blue paper to help them stand out in a pile of plain paper. And today on the Web, the page background is blue.

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