Network Data: Incremental and Differential Backups

When backing up all your data every day is not practical you will have to start being selective about your data backups and schedules, thus incremental and differential backups.

Incremental backups

An incremental backup backs up only those files that were modified since the last time you did a backup. Incremental backups are a lot faster than full backups because your network users probably modify only a small portion of the files on the server in any given day.

As a result, if a full backup takes three tapes, you can probably fit an entire week’s worth of incremental backups on a single tape.

When an incremental backup copies each file, it resets the file’s archive bit. That way, the file will be backed up again before your next normal backup only when a user modifies the file again.

Here are some thoughts about using incremental backups:

  • The easiest way to use incremental backups is the following:

    • A normal backup every Monday

      If your full backup takes more than 12 hours, you may want to do it on Friday so that it can run over the weekend.

    • An incremental backup on each remaining normal business day (for example, Tuesday, Wednesday, Thursday, and Friday)

  • When you use incremental backups, the complete backup consists of the full backup tapes and all the incremental backup tapes that you’ve made since you did the full backup.

    If the hard drive crashes and you have to restore the data onto a new drive, you first restore Monday’s normal backup and then you restore each of the subsequent incremental backups.

  • Incremental backups complicate restoring individual files because the most recent copy of the file may be on the full backup tape or on any of the incremental backups.

    Backup programs keep track of the location of the most recent version of each file in order to simplify the process.

  • When you use incremental backups, you can choose whether you want to

    • Store each incremental backup on its own tape.

    • Append each backup to the end of an existing tape.

Differential backups

A differential backup is similar to an incremental backup except that it doesn’t reset the archive bit when files are backed up. As a result, each differential backup represents the difference between the last normal backup and the current state of the hard drive.

To do a full restore from a differential backup, you first restore the last normal backup and then you restore the most recent differential backup.

For example, suppose that you do a normal backup on Monday and differential backups on Tuesday, Wednesday, and Thursday, and then your hard drive crashes Friday morning. On Friday afternoon, you install a new hard drive. To restore the data, you first restore the normal backup from Monday. Then, you restore the differential backup from Thursday. The Tuesday and Wednesday differential backups aren’t needed.

The main difference between incremental and differential backups is that

  • Incremental backups result in smaller and faster backups.

  • Differential backups are easier to restore.

    If your users often ask you to restore individual files, consider using differential backups.