Backup: Selecting a Backup Type

Selecting a Backup Type

Depending on the backup utility you choose, you will usually have at least three different types of backups that you can use (sometimes more). These include the following:

• Full In a full backup, all specified files are backed up, regardless of whether or not they’ve been modified since the last backup. After being backed up, each file is flagged as having been backed up.

• Incremental During an incremental backup, only the files that have been modified since the last backup (full or incremental) are backed up. After being backed up, each file is flagged as having been backed up.

• Differential During a differential backup, only the files that have been modified since the last full backup are backed up. Even though they have been backed up during a differential backup, the files involved are not flagged as having been backed up. 

To determine your backup strategy, you need to select from the preceding backup types. For example, you could run a full backup every time. This strategy is thorough and exhaustive. It’s also the fastest strategy when you need to restore data from a backup.
However, full backups can take a very long time to complete. This is because every single file is backed up regardless of whether or not it was changed since the last backup. Therefore, many administrators mix full backups with incremental or differential backups.

. If you use a full/incremental strategy, you run a full backup once a week, usually when the sys- tem load is lightest, such as Friday night. Then you run incremental backups each of the other six days in the week. Using this strategy, you should end up with one full backup and six incremental backups for each week. The advantage of this strategy is primarily speed. Because incrementals only back up files that have changed since the last full or incremental backup, they usually run very fast. The drawback to this strategy is that you must restore six backups in exactly the correct order. The full backup is restored first, followed by the first incremental, then the second incre- mental, and so on. This can be a slow process.
Alternatively, you can also use full backups with differentials. In this strategy, you run a full backup on Friday night, usually. Then you run a differential each of the other nights of the week. Remember that a differential backup only backs up files that have changed since the last full backup, not since the last differential. Therefore, each day’s backup gets progressively bigger. The main advantage to this strategy is that restores are really fast. Instead of seven backups to restore, you only have to restore two—the last full backup first, followed by the last differential backup (and no others!). The disadvantage to this method is that the differential backups start out very fast but can become almost as long as a full backup by the time you reach the last day in the cycle.

Whichever strategy you choose, you should be sure to keep a rotation of backups. Many administrators will rotate their backup media so that they have three to four weeks’ worth of past backups on hand. You never know when a file that was deleted two weeks ago will suddenly be needed again!

In addition, you should be sure to verify your backups. Most backup utilities provide you with the option of checking your backup after it’s complete against the original files. If you don’t, you may have errors in your backup.