Developing a Backup Strategy

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To effectively back up a network, you must plan your approach to this complex task. Network backups are more complicated than simply putting a tape in the drive and starting up the software. Your backup strategy should address all of the following questions:

• How much data do you have to back up?
• How much time do you have to perform backups?
• How often should you back up the data?
• Who is going to be responsible for seeing that backups are completed?
• How many tapes (or other media) do you plan to use?
• How often will you overwrite your tapes?

The Backup Window

The concept of the backup window should determine which devices you purchase for backing up your network and which types of backups you'll perform. The backup window is the amount of time you have available to perform backups of your data. You should compare the length of your backup window with the amount of data you have to back up to determine the optimum backup rate for your network. If, for example, your organization works overlapping shifts, leaving only a few hours of network time during which to perform backups, you might have to purchase faster equipment or run several devices in parallel to back up all of your data in the time allotted.

Backup Types

Part of creating a strategy to fit your available backup window involves selecting the type of backups you'll perform. Windows 2000 Backup supports five types of backup jobs that specify how much of your data will be backed up during each job. By selecting the appropriate job type, you can minimize the number of tapes (or other media) and the amount of time required to perform your backups without compromising the safety of your data.

Most of these backup types depend on the archive attribute to determine when the files on a given disk have changed and must be backed up again. The archive attribute in Windows 2000 is the same as that in MS-DOS, no matter which file system you're using. The attribute is a single bit included in the directory entry for each file, which the backup software can set or clear as needed.

Typically, a backup program will clear the archive attributes for all of the files it backs up during a particular job. When you modify a file later, the system automatically sets the attribute as it writes to the disk. This enables the backup software to examine the archive attributes during the next job and back up only the files for which the attribute is set—that is, the files that have changed since the last backup. The backup types described in the following sections are variations on this technique.

Normal Backup

A normal backup, in Windows 2000 parlance, is a full backup of all of the files and directories you select in the Windows 2000 Backup software. As part of the job, the program clears the archive attribute on each file. This type of job is the baseline for future jobs that back up only the modified files.

Incremental Backup

During an incremental backup, the program examines the archive attributes and backs up only the files that have changed since the last normal or incremental backup. As with a normal backup, this type of job also clears the archive attribute on each file it copies. Incremental backups use the minimum amount of tape and also save time by not copying all of the files that remain unchanged during every job. However, performing a restore is inconvenient.

For example, if you perform a normal backup on Monday and incremental backups on Tuesday through Friday, you must restore from all five of these tapes in the order in which they were written to ensure that you have the most current version of every file. If a particular file is updated daily, Windows 2000 Backup will overwrite it with a newer version during the restoration of each tape. However, if you restore only the Monday and Friday tapes, because they represent the last normal backup and the most recent incremental back up, you will lose the most current versions of files that were modified on Tuesday through Thursday, but not on Friday.

Differential Backup

A differential backup is identical to an incremental backup except that the program doesn't clear the archive attributes for the files that it copies to tape. This means that during each differential backup you are copying every file that has changed since the last normal or incremental backup. Thus, after a normal backup on Monday, a differential backup on Tuesday will copy all of the files that have changed (just like an incremental job). However, the differential backups performed on Wednesday through Friday will copy all of the files changed since Monday's normal backup. In other words, some redundancy of data is likely during this kind of job because a file modified only once on Tuesday will be copied during each day's differential backups.

This type of job requires more tape than using incremental jobs, and more time as well, but the advantage is that when you perform a restore, you need only the tapes containing the last normal backup and the most recent differential. Thus, if you have to rebuild a system on Saturday, you need restore only the normal backup from the previous Monday and the most recent differential backup from Friday.

A network backup strategy will typically use incremental or differential backups in addition to normal jobs, but not both. If you're faced with a lot of data to back up and a limited backup window, incrementals are faster and more economical. However, if you have to perform frequent restores, differentials make the process far easier.

Daily Backup

A daily backup copies only the files that have changed on the day that the backup job is performed, disregarding the current state of the archive attribute. This type of job also doesn't clear the archive attributes of the files it copies as it runs. Daily jobs are useful when you want to perform an extra backup on a given day without disturbing an established backup strategy by modifying the archive attributes.

Copy Backup

A copy backup job is the equivalent of a normal backup, except that the program doesn't clear the archive attributes of the files it writes to the tape or other backup medium. You can use a copy backup job to perform an extra full backup without disturbing the archive attributes used by an established backup strategy.

Media Rotation

A media rotation scheme dictates how many tapes (or other media) you will use for your backups. In most cases, you'll want to keep copies of your backups for a while in case you need to perform a restore from them, but eventually, they will become obsolete and you can reuse the tapes. For example, a small network might use a total of five tapes to perform a full backup each weekday and reuse the same tapes each week. In contrast, a large, security-conscious organization might use new tapes for every backup and permanently archive all of the used ones. Most media rotation schemes fall somewhere between these two extremes.

One popular rotation scheme is known as the grandfather-father-son method because it uses three "generations" of tapes representing monthly, weekly, and daily backups, respectively. In this rotation scheme, you perform a full backup every month and retain the tape for a year (preferably off-site); this is the "grandfather." You also perform a full backup every week and retain the tape for a month; this is the "father." The "son" backups are performed daily and retained for a week. The daily jobs can be either full, incremental, or differential backups.

The point of a media rotation scheme is to ensure that you always have a current copy of your data on tape and to reuse the tapes in an even and organized manner. Be sure to label your tapes carefully and store them in a safe place, away from magnetic fields and other adverse environments. We also strongly recommend that you store a copy of your backups off-site, such as in a safety deposit box or other fireproof vault, so that in the event of a true disaster such as a fire, your data is protected.

NOTE

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Some of the third-party network backup products can automatically implement a customizable rotation scheme by tracking the tapes, the number of times they're used, and the names that you should put on the labels. These programs will also tell you which tape to put in the drive each day and let you know which tapes you must use to restore particular files. Unfortunately, Windows 2000 Backup lacks this feature.