Difference between differential and incremental backups

Rather than creating a full backup during each backup schedule you can choose to do an incremental or differential backup. Both of these options ensure only the new or modified files are backed up which saves time and space but it is important to understand what these two options offer and how they might fit in your backup routines.

The archival bit

In the NTFS file system there is an archival bit for each file, this is used by compatible backup systems to mark the file as backed up.

When a file is created or modified it’s archival bit is ‘turned on’, indicating that it needs to be included in a backup. Depending on which backup method is used different tasks can happen to the files and it’s archival bit.

As the archival bit is a part of the NTFS file system it will be working whether or not any backup software is installed or functioning.

What a full backup does

A full backup (sometimes called a normal backup) copies all selected files and resets the archival bit. This type of backup is required for the first backup in a backup routine.

What a differential backup does

A differential backup will copy all files where the archive bit is ‘turned on’, it does not reset the archival bit – this means that in a backup routine where this is used daily the ‘turned on’ files will be copied each day until the archival bit is reset again.

Differential backups are quite often used in the daily backups, and then a weekly full backup is used to reset the archival bit (and have a full complete backup for that point in time).

This sort of routine means that to do a restore only the last normal and last differential backups are required.

What a incremental backup does

Incremental backups are similar to differential backups in the way that they both only copy the files which have been created or modified since the last backup. However, the incremental backup does reset the archival bit.

A backup routine which utilises daily incremental backups will require much less space BUT when a restore is required the last normal and all incremental backups will be required – this can add to the time required to do a restoration.

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