Incremental Backup: What it is and How it Works

Incremental backup is a backup method that only copies data that’s been created or modified since the previous backup. Incremental backup is used when there is a large amount of data to be protected – only copying new or changed data saves time and storage space, making it a common method for online and cloud backup services.

The other main backup types are:

• Full backup. As its name suggests, full backups create a copy of all your files and folders. They are time-consuming to complete and can require a lot of storage space. Full backups are the starting point for both incremental and differential backups
• Differential backup. Only data that has been created or changed since the last full backup is copied. So, if a full backup is done on a Monday, Tuesday’s differential backup will copy everything that’s changed since the previous day, then Wednesday’s backup will also copy everything that’s changed since Monday – including Tuesday’s changes

The key difference between incremental backup and differential backup is that incremental backup copies new or modified data from the last backup of any kind – full or incremental – while differential backup copies new or modified data since the last full backup.

How incremental backup works

A good way of understanding how incremental backup works in practice is to take a working week as an example:

• Monday. The backup software performs a full backup of all files and folders. The backup is then compressed, encrypted and sent to the backup server. This may take some time to transfer, depending on the amount of data that needs backing up
• Tuesday. The backup software scans the backup selection and copies only new files and changes to existing files. You now have a second version of the changed files
• Wednesday. The backup software scans the backup selection and sends only the files that have been created or modified since Tuesday. You now have a second and third version of the changed files, as well as a backup of any new files
• The process is repeated each day, so after 30 days, you’ll have 30 different versions of your modified files – and can restore any of those files from any of those backup dates

Types of incremental backup

There are also different types of incremental backup. Below, we take a look at these different types and briefly explain how they work.

• Synthetic full backup. This works by combining the last full backup with data taken from subsequent incremental backups to create a “synthetic” complete backup. This method has all the advantages of a true full backup while taking up less time and space
• Reverse incremental backup. A reverse incremental backup starts with a full backup and when the next incremental backup is made, the changes are applied to the previous full backup and a new full backup is generated. The next incremental backup is therefore made by capturing the changes against that second full backup, and a third full backup is created. The result of reverse incremental backup is that you always have an up-to-date full backup available, as new and modified data is written into the full backup automatically
• File-level incremental backup. This suits small datasets, with your backup software scanning all your files to see which have been updated since the last backup, then backing only those ones up. You can select the files you want to back up
• Block-level incremental backup. Unlike file-level backup, block-level incremental backup only sends the changed part of your files to the backup repository, rather than complete files and folders
• Byte-level incremental backup. Even more granular than file and block-level backup, byte-level backup only backs up individual bytes that have changed
• Incremental forever backup. This type of incremental backup is disk-based. After an initial full backup is made, only new and modified data is added. The incremental changes can easily be applied to the disk-based full backup

Advantages of incremental backup

• Faster backups. As incremental backup only copies small amounts of data, it’s quicker than full and differential backup and can be performed at frequent intervals
• More efficient storage. Data saved in smaller increments means it takes up less space
• Different versions of files. You can access and restore previous versions of files that have been incrementally backed up, which helps to protect you from data loss caused by accidental deletion

Disadvantages of incremental backup

There are some minor disadvantages to incremental backup it’s important to bear in mind:

• Slower restoration. The time saved in backing up the data gets added to the time needed to restore it. To give you a complete data set, you need to restore the last full backup, then each following incremental backup
• Possible incomplete restoration. In the unlikely event that any of the backups have failed, there is a chance you may not be able to recover all the data you need. A way of safeguarding against this is to arrange for an occasional differential backup to be done

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Editor’s Note: This post was originally published in November 2010 and has been updated for accuracy and comprehensiveness.