What is a Dif­fer­en­tial Backup?

Con­cep­tu­ally, dif­fer­en­tial backups sit between full and in­cre­ment­al backups. A dif­fer­en­tial backup is the middle ground in terms of ef­fi­ciency and com­plex­ity. However, the majority of backups used these days are in­cre­ment­al.

A dif­fer­en­tial backup is a type of backup often used as part of a backup strategy. The first time a dif­fer­en­tial backup is run, a full backup is created. Sub­sequent backups save only changes made to files to the full backup. The first dif­fer­en­tial backup is in­dis­tin­guish­able from the first in­cre­ment­al backup. The top backup options only differ from the next backup onward.

To compare the changes made to files, various methods are used. The simplest scenario is a com­par­is­on of file timestamps from the last full backup and its current state. If the timestamp of a file in the current state is younger, the file has been modified since the last full backup and must be backed up again. Higher precision is achieved by cal­cu­lat­ing hash values from the contents of the two file versions. If the two hashes differ, the file has been modified and will be included in a dif­fer­en­tial backup.

To re­con­struct a dif­fer­en­tial backup, the last full backup is restored first. The changes contained in the dif­fer­en­tial backup are then imported. Any data present in the full backup is over­writ­ten in the process. The result is a mirror of the dataset at the time of the dif­fer­en­tial backup.

Dif­fer­en­tial backups are a good com­prom­ise between full and in­cre­ment­al backups in terms of ef­fi­ciency. The com­plex­ity of creation and recovery is also lower for dif­fer­en­tial backups than for in­cre­ment­al backups and slightly higher than for full backups.

First of all, regularly created dif­fer­en­tial backups are more efficient than re­peatedly creating full backups. That is because only the data that has changed since the last full backup is backed up. At the same time, dif­fer­en­tial backups are more robust than in­cre­ment­al backups. Unlike in­cre­ment­al backups, no chain of related backups is created. Instead, each dif­fer­en­tial backup refers only to the last full backup. This minimises the risk of the entire backup chain becoming unusable if a single link breaks.

To restore from a dif­fer­en­tial backup, only two data sets are needed: the dif­fer­en­tial backup and the as­so­ci­ated full backup. Due to their low com­plex­ity, dif­fer­en­tial backups can be created and restored using on-board tools without the need for spe­cial­ised software.

The dis­ad­vant­age of a dif­fer­en­tial backup is that both the last full backup and the dif­fer­en­tial backup are required for a restore. If the full backup is corrupted, the dif­fer­en­tial backup is largely worthless.

Creating suc­cess­ive dif­fer­en­tial backups of the same full backup results in high re­dund­ancy of the backed-up data. For example, a dif­fer­en­tial backup from Thursday contains the changes from Monday, Tuesday, and Wednesday in addition to the full backup. It follows that the dif­fer­en­tial backup from Thursday is larger than the one from Wednesday and smaller than the one from Friday.

In general, a dif­fer­en­tial backup is suitable if the data volume doesn’t grow too much or if there are short intervals between the in­di­vidu­al dif­fer­en­tial backups. To create and restore backups in an easy manner, without the use of spe­cial­ist software or hardware, a dif­fer­en­tial backup is a good choice.

A tra­di­tion­al backup strategy used by busi­nesses tends to include a full backup every weekend and daily dif­fer­en­tial backups. Following the 3-2-1-backup rule, at least one of the backups should be in the cloud. Due to large data volumes, in­cre­ment­al cloud backups are now more wide­spread.

The most widely used backup software today can create in­cre­ment­al backups. For example, to backup on a Mac, you can use the built-in software ‘Time Machine’ which generates in­cre­ment­al backups. Creating a backup in Windows 10 is very similar, re­gard­less of whether you’re using the built-in Windows backup or the popular tool Robocopy Backup. In both cases the backups are created in­cre­ment­ally.

To create server backup with rsync, you can create a dif­fer­en­tial backup. The result is a dif­fer­en­tial backup that contains the changes since the last full backup in a separate directory. Let’s take a look at the in­di­vidu­al steps:

First, a full backup is created:

The full backup is located in the directory target-path/source-dir/.

1. A dif­fer­en­tial backup of the changes is created af­ter­wards:

The amended files are located in the directory diff-target/.

To restore data, it is enough to reverse the process. Again, two Rsync copy op­er­a­tions are required:

1. Copy a full backup back to the source:

1. Merge dif­fer­en­tial backup with full backup: