Which type of backup is used to backup only files that have changed since the last full backup?

The main difference between incremental and differential backups is how they save space and time by storing only changed files. However, the effectiveness of incremental backups is very different from differential backups.

Before we dive into the differences between Incremental and Differential backups let's review Full Backups. 

Are you still using Full Backups?

In its simplest form, a full backup is the starting point for both Incremental and Differential backup strategies. A full backup, of course, backs up all the data on a partition or disk by copying all disk sectors with data to the backup image file. 

A full backup is time-consuming and requires a lot of space, it’s usually used as part of a backup plan with long intervals, like once a week or a month. During this time, if anything goes wrong, a lot of data can be lost. That’s when these two backup strategies come in hand.

An average partition or disk only contains a small number of changes per day, or even per week. Therefore, it makes sense only to backup data that has changed on a daily basis. This is the basis of smart backup strategies.

Incremental vs Differential Backup - How do they compare?

Differential and incremental backups are different backup strategies with the same purpose: optimize backup time and space. Differential backups only back up the files that have changed since the previous full backup, while incremental backups do the same, they back up the files that have changed since the previous incremental or full backup.

Datto SIRIS ensures reliable backups with our Inverse Chain Technology™ with fundamentally changes the way backup and recovery is done allowing for every incremental snapshot to be a fully constructed recovery point.

Learn more about Datto SIRIS

What is Differential Backup?

Differential backup strategy backs up files and folders that have changed since the last full backup, on a daily basis. They are much quicker than full backups since less data is being backed up.

One of the benefits of this strategy over incremental backup is that you only need the last full backup and last differential backup to restore data, making the restoration process much faster. However, the amount of space consumed by backed up data will grow with each differential backup until the next full backup.

Even though differential backups are more flexible than full backups, they still present too many obstacles for routine use, particularly as the next full backup gets closer.

Diagram Showing How Differential Backups Work

What is Incremental Backup? 

Unlike differential backups, incremental backup copies changed files since the last backup of any type, which can be a full backup or an incremental backup. When incremental backups are performed, the shorter the time interval between backups, the less data needs to be backed up.

It’s the strategy with the most space efficiency between the three: full, differential, and incremental.

In spite of incremental backups giving greater flexibility and granularity (time between backups), they have a reputation for taking longer to restore since they must be constructed from the last full backup and all subsequent incremental backups.

Datto has developed Inverse Chain Technology™ that uses the best of all these backup strategies.

Smart backup with Inverse Chain Technology

Datto Inverse Chain Technology ensure that you can quickly and reliably backup and recover your data in moments. 

This is built off this traditional incremental backup, however, Datto uses ZFS to make each incremental point refer to every other point in the chain therefore only one full backup needs to be stored. 

This increases the backup chains redundancy and the flexibility in how each point is stored and used to virtualise a system in a disaster recovery scenario.   

Speak to our Sales team about Datto SIRIS & our Inverse Chain Technology today 

Full vs. incremental vs. differential backup

Incremental, differential, and full backups are common techniques. Less frequently encountered types include forever-incremental, synthetic, and mirror backups.

Full backups: Full backups are complete copies of all configured data. This backup is best used periodically, although it is essential to have all data entirely backed up, because creating and implementing a full backup regularly consumes far more storage, time, network bandwidth, and other resources.

Incremental backups: Incremental backups save resources and time because they back up only the data that changed since the last backup of any kind. This consumes less time and storage space but makes restoration more difficult as it means restoring both the last incremental backup and the last full backup as well.

A full backup and a level 0 incremental backup are physically identical, with one difference: the level 0 backup can be used as the parent for a level 1 backup because it is saved in the RMAN repository, an online backup source of read/write datafiles, as an incremental backup.

Differential backups: Differential backups, also called cumulative incremental backups, also save resources and time because they backup only the data that changed since the last full backup — but in this case, where the incremental backup goes back to the last backup of any type, the differential backup restores data back to the last full backup only.

Comparing full, incremental and differential backups, the full backup is just like it sounds; the incremental backup covers just the most recent changes from the last backup of any type; and the differential backup goes back to the last full backup.

Backup systems perform full backups in many different ways. An incremental-forever approach performs just one full backup ever, followed by incremental backups. Block-level incremental-forever backups, a subset of this type, store the backup on the changed blocks rather than all files that have been created or modified since the last backup.

The synthetic full backup process can also reconstruct a full backup by copying data from existing backups. A synthetic full backup uses only the backup server and its storage; it consumes no resources of the system it is backing up.

Similar to the full backup, a mirror backup creates an exact copy of the source data set. However, while a classic full backup will track different versions of the files, in a mirror backup, only the latest data version is stored in the backup repository; the mirror backup copies only modified files. All individual backup files are stored separately, just as they are in the source, not in a single compressed/encrypted container file.

Mirror backups allow for quick, direct access to individual backup files without a formal restore operation. However, they also demand large amounts of storage, and present a high risk of data loss, data corruption, misuse, and unauthorized access. Furthermore, mirror backup files are actual mirrors, meaning they reflect what happens in the source files. Any adverse modifications to the source due to accident, human error, malware action, or sabotage may produce the same outcome in the mirror backup.