In this discussion on incremental vs differential backup, we define the two terms using definitions that most people in the industry agree upon. There are complications in terminology, however, and we will look at those issues as well.
Differential backups copy those files that have been changed since the last full backup took place. So if a full backup was done on Day 1, Day 2's differential will copy all of the files that have changed since Day 1's backup copied everything. Day 3's differential backup will also copy all of the files that have changed since Day 1's full copy was made.
The key advantage of differential backups comes when data needs to be restored. Because a full backup was taken and the differentials copied everything that subsequently changed, only the full backup and the latest differential need to be restored.
The main disadvantage is that the size of the differential copy increases each time a backup is taken until the next full version is made, which can begin to impinge on backup window duration.
Incremental backups copy all of the files that have changed since the last backup was made. They do this whether the last backup was a full one or an incremental copy. So if a full backup was done on Day 1, Day 2's incremental will back up all of the files that have changed since Day 1. Likewise, Day 3's incremental backup will only copy those files that have changed since Day 2's incremental took place.
The main advantage to incremental backups is that fewer files are copied in the period between full backups, which means you will get a shorter backup window. The main disadvantage is that when you want to carry out a complete restore, the most recent full backup and all of the subsequent incremental copies must be restored. This can make the restore process a lengthier one than when using a full backup plus the most recent differential copies only.
Technology and terminology
Although I have used the terminology that most people in the industry agree upon when talking about incremental vs differential backup, there are some variants in usage that can cause confusion. So let's define what the difference backup types do: full, incremental and differential backup.
Obviously, a full backup copies all files. After that, subsequent backups between full copies are carried out with reference to either the last full backup or the last partial backup. Technically speaking, the difference between backup types is often based on their relationship to the archive bit. The archive bit is a file component that is activated when a file is created or changed. It indicates whether the file has been backed up since it was last modified.
A full backup resets the archive bit. This means any files changed after that will be copied by backups between that full copy and the next one. In contrast, a differential backup copies those files that have changed since the full backup. However, it does not reset the archive bit, meaning it will copy them again the next day. An incremental backup copies changed files and resets the archive bit so that it does not copy the file unless it changes again.
Most backup products use the archive bit to determine whether or not a file should be copied; however, some products use a time stamp or database to track changes.
Watch out for variants in the terminology used. Some people describe anything other than a full backup as a form of incremental. On this basis, a cumulative incremental backup is what I have termed a differential backup, while a differential incremental backup is what I have referred to here as simply incremental.