A differential backup is a type of data
backup
In information technology, a backup, or data backup is a copy of computer data taken and stored elsewhere so that it may be used to restore the original after a data loss event. The verb form, referring to the process of doing so, is "back up", w ...
that preserves data, saving only the difference in the data since the last
full backup
In information technology, a backup, or data backup is a copy of computer data taken and stored elsewhere so that it may be used to restore the original after a data loss event. The verb form, referring to the process of doing so, is "back up", w ...
. 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
An incremental backup is one in which successive copies of the data contain only the portion that has changed since the preceding backup copy was made. When a full recovery is needed, the restoration process would need the last full backup plus al ...
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.
Meaning
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.
[''SQL Server differential backups.''](_blank)
Carlos Rojas. EMC Community Network. EMC Corporation. 2 March 2011. Retrieved 21 August 2012. 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.
Illustration
The difference between incremental and differential backups can be illustrated as follows:
;''Incremental backups:''
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
Rotation, or spin, is the circular movement of an object around a '' central axis''. A two-dimensional rotating object has only one possible central axis and can rotate in either a clockwise or counterclockwise direction. A three-dimensional ...
.
;''Differential backups:''
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. For example,
Oracle Corporation
Oracle Corporation is an American multinational computer technology corporation headquartered in Austin, Texas. In 2020, Oracle was the third-largest software company in the world by revenue and market capitalization. The company sells da ...
uses a backward description of differential backups in their DB product as of May 14, 2015:
"Differential incremental backups - In a differential level 1 backup, RMAN backs up all blocks that have changed since the most recent cumulative or differential incremental backup, whether at level 1 or level 0. RMAN determines which level 1 backup occurred most recently and backs up all blocks modified after that backup. If no level 1 is available, RMAN copies all blocks changed since the level 0 backup."
See also
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Backup rotation scheme
A backup rotation scheme is a system of backing up data to computer media (such as tapes) that minimizes, by re-use, the number of media used. The scheme determines how and when each piece of removable storage is used for a backup job and how long ...
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Continuous data protection Continuous data protection (CDP), also called continuous backup or real-time backup, refers to backup of computer data by automatically saving a copy of every change made to that data, essentially capturing every version of the data that the user s ...
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Delta encoding
Delta encoding is a way of storing or transmitting data in the form of '' differences'' (deltas) between sequential data rather than complete files; more generally this is known as data differencing. Delta encoding is sometimes called delta compre ...
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Disk Archive - portable robust program for archiving and backup
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Dump (Unix)
The dump command is a program on Unix and Unix-like operating systems used to back up file systems. It operates on blocks, below filesystem abstractions such as files and directories. Dump can back up a file system to a tape or another disk. I ...
- UNIX utility for multilevel incremental file system backups.
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rsync
rsync is a utility for efficiently transferring and synchronizing files between a computer and a storage drive and across networked computers by comparing the modification times and sizes of files. It is commonly found on Unix-like operat ...
- File synchronization algorithm and protocol.
References
Further reading
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{{DEFAULTSORT:Differential Backup
Data synchronization
Incremental computing
Backup