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Backup 101: Incremental vs differential backup

In this Backup 101 article, we look at incremental versus differential backup and walk through the advantages and disadvantages of these two key backup strategies

Incremental or differential backup: Which should you choose? In this article, we take a look at these two key concepts in backup, looking at the pros and cons of each, as well as point to resources that can help with other backup methods, such as synthetic full and incremental forever backups.

The most fundamental type of backup is a full backup, which copies all data in a given system. Differential and incremental backups are defined by how they build on that full backup.

A differential backup copies files that have changed since the last full backup was taken. So if a full backup was done on Monday, the differential on Tuesday will copy files changed since Monday’s backup copied everything. Then, Wednesday’s differential will copy everything that changed since Monday’s full backup.

The big advantage of differential backups is that when data needs to be restored it can be built from the full backup and the latest differential copy.

The key drawback is that the size of the differential backup increases every time one is made until the next full backup job. That can lengthen backup window duration and will impact on storage space needed to store the differentials.

Where differential backups copy everything that changed since the last full backup, incremental backups copy everything that changed since the last backup. So, if a full backup was carried out on Monday, the incremental on Tuesday will will just copy what has changed since Monday. Likewise, Wednesday’s incremental copies only the changed data since Tuesday’s incremental took place.

The main benefit of incremental backups is that less is copied every day than if you carried out differential backups. That means you get a shorter backup window on days between full backups, and less storage space is needed for them.

The key disadvantage of differential backups is that when you want to do a complete restore, it has to be built from the most recent full backup and all incremental between then and the point to which you want to restore.

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Look out also for variants on full, incremental and differential that may be offered by some backup product providers.

These include: Synthetic full, which builds a “full” backup from a real full and subsequent incrementals; incremental forever, which stores fulls and subsequent incrementals to allow restores to chosen points in time, and; reverse incremental, in which a synthetic full is the default, but incrementals are kept to allow rollback to a specified point.

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