A differential backup is a type of data backup that preserves data, saving only the difference in the data since the last full backup. The rationale in this is that, since changes to data are generally few compared to the entire amount of data in the data repository, the amount of time required to complete the backup will be smaller than if a full backup was performed every time that the organization or data owner wishes to back up changes since the last full backup. Another advantage, at least as compared to the incremental backup method of data backup, is that at data restoration time, at most two backup media are ever needed to restore all the data. This simplifies data restores as well as increases the likelihood of shortening data restoration time.
A differential backup is a cumulative backup of all changes made since the last full backup, i.e., the differences since the last full backup. The advantage to this is the quicker recovery time, requiring only a full backup and the last differential backup to restore the entire data repository. The disadvantage is that for each day elapsed since the last full backup, more data needs to be backed up, especially if a significant proportion of the data has changed, thus increasing backup time as compared to the incremental backup method.
It is important to use the terms "differential backup" and "incremental backup" correctly. The two terms are widely used in the industry, and their use is universally standard. A differential backup refers to a backup made to include the differences since the last full backup, while an incremental backup contains only the changes since the last incremental backup. (or, of course, since the last full backup if the incremental backup in questions is the first incremental backup immediately after the last full backup). All the major data backup vendors have standardized on these definitions, including Microsoft, Acronis, and Symantec.
Microsoft, makers of Backup and Restore, defines incremental backups and differential backups as follows: "Incremental: An incremental backup provides a backup of files that have changed or are new since the last incremental backup....Differential: A differential backup provides a backup of files that have changed since a full backup was performed. A differential backup typically saves only the files that are different or new since the last full backup, but this can vary in different backup programs. Together, a full backup and a differential backup include all the files on your computer, changed and unchanged." Microsoft products, such as Exchange Server 2010 use this definition in their literature and support sites.
Acronis, makers of Acronis True Image, states that “A differential backup backs up only the files that changed since the last full back...Incremental backups also back up only the changed data, but they only back up the data that has changed since the last backup — be it a full or incremental backup.”
CA Technologies, makers of ArcServe, states that “Incremental Backup…backs up the… files that have changed since the last backup. Differential Backup… backs up …files that have changed since the last full backup.”
Symantec, makers of Backup Exec, states that “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...A Differential backup will back up all selected files that are new and changed since the last full backup.”
EMC Corporation, makers of EMC NetWorker (formerly Legato), states that “Incremental is a backup of latest changes since the last backup (any level) so when a full recovery is needed you would need to restore the last full backup plus all the incremental [backup]s until the point-in-time you want to restore...Differential backups are incremental backups since the last full backup… the differential backups will always save the differences between the last full.”
Quest Software, makers of NetVault Backup, states that “Incremental – An Incremental backup transfers the changes in a volume since the last Full, Differential, or Incremental backup. Incremental backups consume minimum storage space and are quicker to perform...Differential – A Differential backup transfers the changes in a volume since the last Full backup. Differential backups speed up recovery since the plug-in is only required to restore two savesets, the Full and the latest Differential backup.”
The difference between incremental and differential backups can be illustrated as follows:
- Incremental backups:
|Backup Type >>>||Full||Incremental||Incremental||Incremental||Incremental||Incremental||Incremental||Full|
|Effect >>>||N/A||Changes since Sunday||Changes since Monday||Changes since Tuesday||Changes since Wednesday||Changes since Thursday||Changes since Friday||N/A|
Editor’s Note: the above assumes that backups are done daily. Otherwise, the “Changes since” entry must be modified to refer to the last backup (whether such last backup was full or incremental). It also assumes a weekly rotation.
- Differential backups:
|Backup Type >>>||Full||Differential||Differential||Differential||Differential||Differential||Differential||Full|
|Effect >>>||N/A||Changes since Sunday||Changes since Sunday||Changes since Sunday||Changes since Sunday||Changes since Sunday||Changes since Sunday||N/A|
It is important to remember the industry standard meaning of these two terms because, while the terms above are in very wide use, some writers have been known to reverse their meaning. One case, for example, is the one found at The Elder Geek website.
- Backup rotation scheme
- Continuous data protection
- Delta encoding
- Dump (program) - UNIX utility for multilevel incremental file system backups.
- rsync - File synchronization algorithm and protocol.
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