How Do Delta Backups Work?

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In order to be securely protected, you need to maintain many historical copies of your data for point-in-time recovery. However, these can take up a lot of space unless you use some sort of “delta” or “incremental” backup system.

Incremental backups are different than full backups, because they only copy over the files or data blocks that have recently changed.

There are 3 leading methodologies for performing incremental or delta backups, and it’s important to understand how each one will impact your recovery process before implementing.

To illustrate, we’ll use the example of a company with a 30 day backup cycle. Once per month, they perform a full system backup, and then they only copy over their changed data on a daily basis.

After 30 days, they perform another full backup and start the cycle again from day 1.

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Differential Incremental (Efficient Delta Backups)

With cumulative incremental backups, you only copy the data that has been changed since the previous day. (Or backup cycle) This is a compact and efficient way to perform incremental delta backups.

However, the recovery process can be complex.

If you need to recover on day 28 of the cycle, you need to reload the first full backup, and then load each one of the 27 following incremental before you can begin recovering. This complexity can lead to longer recovery times, and it also creates opportunities for errors and backup failures.

Cumulative Incremental (Fast, Simple Recovery)

With cumulative backups, you only copy the data that has changed since day 1 of the backup process. This approach has the disadvantage that the daily backups will get larger towards day 28 of the cycle. However, the recovery process is greatly streamlined since you only need to load the first full backup copy, followed by the most recent incremental backup.

So when making the choice between differential and cumulative, you need to decide between backup efficiency and recovery performance. But there is another method which offers the efficiency of a differential incremental backup, with the recovery performance of a cumulative incremental backup.

Progressive Backups (Best of Both Worlds)

Progressive backups are often called “incremental forever” backups.

A progressive backup copies over changed data in a way that’s similar to differential incremental backups. However, these backups are then passed along to a special backup server which has the ability to recompile these fragments into a single “full” backup set that can be downloaded any time a recovery is necessary.

Unlike the cumulative and differential approaches, backups do not need to be manually reconstructed as part of the recovery process. Instead, the administrator can request the full backup version from the backup server. This is an incredibly efficient and error-free way to perform emergency recoveries.

As an added benefit, the progressive paradigm does away with the need to perform full system backups every thirty days. Instead, you perform one full backup on day 1, and copy over incremental changes forever. The backup software is intelligent enough to track and organize these changes in an efficient way, and to rebuild your data from any point in time when you need it.

To learn more about differential incremental, cumulative incremental and incremental forever, we’ve included this video on delta backups.

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