What are the differences between Differential and Incremental backups?

What are the differences between Differential and Incremental backups?

  • Article ID:100020854
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  • Product(s):Backup Exec


Differences between Differential and Incremental backups.


Before explaining the differences between Differential and Incremental backups, it is necessary to understand how Backup Exec (tm) knows what files have changed since the full backup.

New and Changed File Backup Schemes:

Rather than performing a full backup each time, only those files which are new and changed can be backed up. This can save backup time and reduce the media required. There are two different kinds of new and changed file backups: Incremental and Differential.  

To explain the difference between the two, it is necessary to understand a basic on-off file attribute called the archive bit. Incremental and Differential backups are dependent upon the archive bit/flag of the files being backed up. If you were to view the properties of a file, you would be able to see whether the archive bit is checked.

The archive bit/flag is either checked or unchecked. When checked, the operating system is indicating that the file needs to be backed up. If the archive bit is unchecked, the OS will automatically check the archive bit of any file that is modified by any application.

A Full backup, also known as an "all selected files" backup, backs up all of the selected files and sets the archive bit to OFF (unchecked), thus indicating that these files have been backed up. A Full backup will back up all the selected files, regardless whether the archive bit is ON or OFF.

An Incremental backup backs up only the selected files that have their archive bit set to ON, setting them back to OFF. This results in a backup of all files that are new or changed since the last backup, whether it was a full or an incremental. The advantage of an Incremental is that it takes the least amount of time and media of all the backup methods.

A Differential backup backs up only the selected files that have their archive bit set to ON but does not set the archive bit back to OFF. A Differential backup will back up all selected files that are new and changed since the last full backup. The advantage of a Differential comes at restore time; you'll need only the last full backup and the last differential to get a complete restore. In the case of restoring with Incremental backups, all the Incremental backups since the last full backup plus the last full backup would be necessary.
Here are the Backup Methods simplified.

A very simple two media weekly rotation that takes advantage of new and changed files might go like this:
Media 1: labeled "Week 1 Full"
Media 2: labeled "Daily Differential 1"


Create three backup jobs with the same file selections:
Monday: Full backup, overwrite. Week 1 Full.
Tuesday through Friday : Differential backup, overwrite. Daily Differential.
(If the differential is too large to append to the media, then add an additional media and label it "Daily Differential 2.")

It is a good idea to have and rotate more than one pair of media sets. Four pairs of media will provide almost a month's worth of in-depth data protection for most home and small office applications.

Note: "Shadow Copy Components" and "System State" can´t be backed up using either Incremental or Differential, and Backup Exec will automatically use the Full method if either of these are included in the selection list, regardless the backup method chosen in the job settings. 


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