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Data backup types explained: Full, incremental, differential and incremental-forever backup

What you will learn in this tip: Learn about full backups, incremental and differential backups, and newer types of backups like synthetic and incremental-forever backups. You will also learn how to choose the best data backup type

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for your organization's data.

For quite some time, there have been three basic types of backups: full, incremental and differential. More recently though, data backup software vendors have introduced some newer types of backups that you might not be familiar with. Before I do though, I am going to give you a quick crash course in full, incremental and differential backups.

Full backups

A full backup is exactly what the name implies. It is a full copy of your entire data set. Although full backups arguably provide the best protection, most organizations only use them on a periodic basis because they are time consuming, and often require a large number of tapes or disk.

Incremental backup

Because full backups are so time consuming, incremental backups were introduced as a way of decreasing the amount of time that it takes to do a backup. Incremental backups only backup the data that has changed since the previous backup.

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Learn about the benefits and drawbacks of incremental and differential backup

Read more about incremental and differential backups

For example, suppose that you created a full backup on Monday, and used incremental backups for the rest of the week. Tuesday's backup would only contain the data that has changed since Monday. Wednesday's backup would only contain the data that has changed since Tuesday.

The primary disadvantage to incremental backups is that they can be time-consuming to restore. Going back to my previous example, suppose that you wanted to restore the backup from Wednesday. To do so, you would have to first restore Monday's full backup. After that, you would have to restore Tuesday's tape, followed by Wednesday's. If any of the tapes happen to be missing or damaged, then you will not be able to perform the full restoration.

Differential backups

A differential backup is similar to an incremental backup in that it starts with a full backup, and subsequent backups only contain data that has changed. The difference is that while an incremental backup only includes the data that has changed since the previous backup, a differential backup contains all of the data that has changed since the last full backup.

Suppose for example that you wanted to create a full backup on Monday and differential backups for the rest of the week. Tuesday's backup would contain all of the data that has changed since Monday. It would therefore be identical to an incremental backup at this point. On Wednesday, however, the differential backup would backup any data that had changed since Monday.

The advantage that differential backups have over incremental is shorter restore times. Restoring a differential backup never requires more than two tape sets. Incremental backups on the other hand, may require a great number of tape sets. Of course the tradeoff is that as time progresses, a differential backup tape can grow to contain much more data than an incremental backup tape.

Synthetic full backup

A synthetic full backup is a variation of an incremental backup. Like any other incremental backup, the actual backup process involves taking a full backup, followed by a series of incremental backups. But synthetic backups take things one step further.

What makes a synthetic backup different from an incremental backup is that the backup server actually produces full backups. It does this by combining the existing full backup with the data from the incremental backups. The end result is a full backup that is indistinguishable from a full backup that has been created in the traditional way.

As you can imagine, the primary advantage to synthetic full backups is greatly reduced restore times. Restoring a synthetic full backup doesn't require the backup operator to restore multiple tape sets as an incremental backup does. Synthetic full backups provide all of the advantages of a true full backup, but offer the decreased backup times and decrease bandwidth usage of an incremental backup.

Incremental-forever backup

Incremental-forever backups are often used by disk-to-disk-to-tape backup systems. The basic idea is that like an incremental backup, and incremental-forever backup begins by taking a full backup of the data set. After that point, only incremental backups are taken.

What makes an incremental-forever backup different from a normal incremental backup is the availability of data. As you will recall, restoring an incremental backup requires the tape containing the full backup, and every subsequent backup up to the backup that you want to restore. While this is also true for an incremental-forever backup, the backup server typically stores all of the backup sets on either a large disk array or in a tape library. It automates the restoration process so that you don't have to figure out which tape sets need to be restored. In essence, the process of restoring the incremental data becomes completely transparent and mimics the process of restoring a full backup.

What backup type is best for you?

As with any backup, it is important to consider which backup type is best suited to your own organization's needs. Ask yourself the following questions:

  1. What does your service-level agreement dictate in regard to recovery time?
  2. What are the policies regarding storing backup tapes offsite? If backups are shipped offsite, incremental backups are a bad idea because you have to get all the tapes back before you can begin a restoration.
  3. What types of backups does your backup application support?

As you can see, synthetic full backups and incremental-forever backups go a long way toward modernizing the backup process, but it's important to make sure you choose the best backup type for your organization's data.

About the author: Brien M. Posey, MCSE, has previously received Microsoft's MVP award for Exchange Server, Windows Server and Internet Information Server (IIS). Brien has served as CIO for a nationwide chain of hospitals and was once responsible for the Department of Information Management at Fort Knox. You can visit Brien's personal website at www.brienposey.com.


This was first published in July 2010

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