(A short version of this article first appeared in Matthew Harris' Disk Compression Book)

The "Grandfather-Father-Son" Back-Up Method

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Copyright © 2012-2013 by Matthew S Harris

Matthew S Harris is a software & database designer, contract programmer & consultant.

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This section explains the tools and materials you need to implement the Grandfather method, how to organize your materials for the Grandfather method, and recommends best practices for your back-ups.

What You'll Need

To perform any kind of data back-up, you need three basic items: a back-up device, a back-up application, and media on which to store your backed-up data.

Assuming a 5-day work week, the Grandfather back-up method uses 10 sets of backup-media. Six sets are used for full back-ups, and four sets for incremental back-ups. For businesses which operate 7-days a week, the Grandfather back-up technique requires 12 sets of backup media: 6 sets for full back-ups, and 6 sets for incremental back-ups.

A full back-up includes all of the files designated for back-up, whether or not they have changed since the last backup. In the Grandfather method, full backups are made once each week.

An incremental back-up includes only the files (out of those designated for backup) which have changed or been created since the last time they were backed-up. In the Grandfather method, incremental back-ups are made once each day, except on days when a full back-up is performed.

The Grandfather back-up method re-uses media sets as you proceed through the rotational schedule. Depending on the degree of thoroughness you choose (see "How Thorough?"), you delete files more than 12 weeks old from each media set in order to make room for additional back-ups, or you simply over-write the last set of back-up files with the current back-up files.

How Thorough?

You may choose to utilize the Grandfather back-up method at two levels of thoroughness.

Which level of thoroughness you choose depends on whether you simply want to be able to recover the most recent status of your files in the event of a major data-loss, or if you have a need to restore specific daily file versions.

Estimating The Amount of Media Required

If you are backing-up for a 5-day work week, you'll need 10 sets of back-up media (12 sets for a 7-day work week). You need 6 media sets for full back-ups, and 4 sets for incremental backup (6 incremental sets for a 7-day work week).

Estimating the amount of media needed for a single full back-up is straightforward: Check the total combined size of the files you want to back-up, and use that figure plus 10% to compute how many tapes or other media you will need. The plus 10% allows for some future growth in the size of the files being backed-up, and should allow successfully for any storage overhead. Many back-up applications allow you to "preview" a back-up, showing you the combined size of the files being copied and how much space they will occupy on the media.

For an incremental back-up, your own experience with how often and how many files change or are created is your best guide. It is best to over-estimate your needs for an incremental back-up, as running out of media in the middle of a back-up operation is more than inconvenient! About 30% to 40% of the media required for a full back-up is a good starting point for an incremental back-up.

Labeling the Back-up Media Sets

After you gather together the backup-media sets you need, label them as follows:

You use one of the three "Friday" media sets on each Friday for the first three weeks of each month (in order, 1, 2, 3). On the last Friday of each month you use one of the "Month" media sets (also in order, 1, 2, 3).

You use the incremental media sets on each day of the week for which the incremental media set is labeled — Monday through Thursday (or Saturday through Thursday, for a 7-day work week).

Off-Site Back-up - Best Practice

The point of making a back-up is to preserve files which may be personally important to you, or important to your business. Simply making copies of your files does not necessarily fulfill these goals.

Apart from accidental deletions and occasional file corruption, typical causes of data loss include disk failure, computer failure, malicious software, or disasters such as fire or flood.

If you merely copy files to another location on the same disk drive, a drive failure or activation of malicious software is likely to take out your back-up files as well as your original data. If you copy files to another hard drive in the same computer, a catastrophic computer failure or malicious software could also destroy your back-up files. Similarly, just making back-ups from one server to another may not guarantee the safety of your back-up copies (what if the building burns to the ground and all of your servers are destroyed?).

The best way to keep your back-up files safe is to store them in a physically remote location (that is, not in the same building housing the data you are backing-up). This practice is referred to as off-site back-up. By storing your backup-media in another location, you help ensure that your back-up copies will not be destroyed by whatever event destroys your original data.

Ideally, all of your back-up media (except the media set for the current day) should be stored off-site. At the end of the day, the most recent back-up set is removed to off-site storage; in the morning, the media set needed for the next back-up is brought in from off-site storage.

You can also accomplish the effect of off-site storage by transferring your backup files electronically (that is, over an intranet or the Internet) to a remote server - provided that server is not housed in the same building/facility as the one where the data you are backing-up is stored.

For computer users at home, the effort of maintaining off-site backup is usually not practical. For home users, using an external USB drive is a good compromise. Leave the external drive disconnected and powered-down when not actually performing a back-up. Because the drive is disconnected and un-powered, it is unlikely to be affected by catastrophic computer failure or malicious software. In the event that some sort of evacuation of the home is required, you can simply grab the external drive and take it with you, thus preserving your backed-up data.

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