An incremental backup is a type of backup that only copies files that have changed since the previous backup.
For example, if a full backup was performed on Monday, Tuesday's incremental will back up all changed files since Monday's backup. However, Wednesday's incremental will only back up files that have changed since Tuesday's incremental backup and so on until another full backup is performed.
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Common types of incremental backups
There are various types of incremental backups. Some of these variations include:
Synthetic full backup: A full backup that is made by reading the previous full backup and subsequent incremental backups rather than reading the data from the primary storage.
Block-level incremental backup: Common form of incremental backup in which the backup software backs up storage blocks that have been written rather than backing up files and folders. Block-level backups are more efficient than file-level backups because only the changed blocks are backed up as opposed to the software having to back up the entire file.
Byte-level incremental backup: Byte-level incremental backups monitor the file system for individual bytes that have changed and then back up those bytes on an incremental basis. This approach yields the smallest possible backups.
Incremental forever backup: Also known as progressive incremental backups, this variation is designed to work on disk-based backup systems. After an initial copy of the data is made, the software only backs up new and modified data. Because the backup is disk-based, there is no need to create periodic full backups.
Multi-level incremental backups: In a multi-level incremental backup, levels are defined as a way to decrease the amount of time it takes to restore a backup. Suppose you create a full backup, defined as Level 0, on Monday. You then create a Level 1 backup on Tuesday and a Level 2 backup on Wednesday. Under normal circumstances, if you created and then restored an incremental backup on Thursday, it would require the backups from Monday, Tuesday and Wednesday to also be restored. As an alternative, a backup administrator might create a Level 1 incremental backup on Thursday, which would include all the data created or modified since the last Level 1 backup was made (in this case on Tuesday). Consequently, a full restoration would require restoring the backups from Monday, Tuesday and Thursday rather than restoring the backups from Monday, Tuesday, Wednesday and Thursday. This technique is sometimes referred to as a differential incremental backup.
Incremental backup pros and cons
The main advantage of incremental backups is that fewer files are backed up daily, allowing for shorter backup windows. The disadvantage is that during a complete restore, the latest full backup and all subsequent incremental backups must be restored, which can take significantly longer than restoring a full backup.