Full, Incremental and Differential
The Three Backup Types Defined
Full incremental differential backup describes three ways of deciding how much data each backup run copies. The types trade storage space and backup speed against how hard a restore is.
- Full backup. Copies all of the data every time. Each full backup is self-contained, so one set restores the whole site.
- Incremental backup. Copies only what has changed since the last backup of any kind, whether that was full or incremental.
- Differential backup. Copies everything that has changed since the last full backup, so each differential grows until the next full backup is made.
Most real schedules mix these types, for example a full backup once a week with incremental or differential runs on the days in between. How often these runs happen is covered in how often to back up a website.
How the Types Compare on Storage, Speed and Restores
Each type makes a different trade-off. The differences matter most when a site is large or changes often.
- Storage. Full backups use the most space, because each one stores the whole site. Incremental backups use the least. Differential backups fall in between, and they grow each day until the next full backup.
- Backup speed. Incremental backups are the quickest to run, since they copy the smallest amount of data. Full backups are the slowest. Differential backups take longer than incremental ones but less time than full ones.
- Restore complexity. A full backup restores in one step. A differential restore needs the last full backup plus the latest differential. An incremental restore needs the last full backup plus every incremental made since, applied in order.
In practice, the choice comes down to which cost you feel first. Full backups cost the most storage and time on every run. Incremental and differential backups save both, but they make restores longer or more dependent on earlier copies.
The set of backups needed for a restore is called the restore chain. A long incremental chain takes longer to restore, and a single damaged link can make the later backups unusable. Differential backups keep the chain shorter, which is why many administrators see them as a reasonable middle ground. Schemes that combine these types over time are covered in grandfather-father-son rotation.
A Week of Backups in Practice
Consider a site backed up with a full copy on Sunday and an incremental or differential copy on each following day. On Wednesday, the site is damaged and must be restored from the Wednesday copy.
With incremental backups, the restore needs Sunday's full copy plus the Monday, Tuesday and Wednesday incremental copies: four sets, applied in order. With differential backups, it needs only Sunday's full copy plus the Wednesday differential, which holds all changes since Sunday. The differential run takes longer each day, but the restore is simpler.
Frequently asked questions
Which Backup Type Is Best for a Website?
There is no single best type. Many sites use a full backup at regular intervals, with incremental or differential backups in between. Choose based on how much the site changes, how much storage you can use and how quickly you need to restore. How many copies to keep is a separate question, covered in backup retention policy.
What Is a Restore Chain?
A restore chain is the set of backups needed, in order, to rebuild a site to a particular point in time. For a full backup, the chain is a single set. For incremental backups, it is the last full backup plus every incremental after it. If one link in the chain is missing or damaged, the backups that depend on it may not be usable.