Understanding Backup: Full, Differential and Incremental Backups Explained

August 12, 2018
Posted in Blog
August 12, 2018 Richard McElroy

Recently a situation arose that showed why understanding the differences between a full, differential and incremental backup and understanding the role of each in a backup strategy is imperative to avoid catastrophe.

Full backups take time and, if there is a lot of data, time and disk space. To reduce the amount of time and disk space a backup uses, backup vendors implemented differential and incremental backups. In the case mentioned above there was a large amount of data that didn’t change often so the owner of the data created a backup relying on incremental backups to save space. When we took over the account we reviewed the backup process. Sure enough the backup ran every day creating an incremental backup in half an hour. Everything appeared fine. No errors or notifications. It was a false sense of security.  The reason why is at the end of the post.

For small businesses that rely on in house backups understanding the types of backups available will help refine your backup strategy.

Full Backup  – As the name implies, a full backup is a complete backup of the disk sectors of the volume or partition being imaged. A complete backup is the simplest type of backup, but it also takes the most time to complete and uses the most storage space.  Typically, in a small business a full backup is performed weekly. However, if the company has a large amount of data they may only run a full backup once a month or possibly more. Because businesses can’t afford to lose a full weeks or months worth of data other backup methods are employed to capture changes to the data for the period between full backups.

Differential Backup – A differential backup backs up only the files that changed since the last full backup. Assume that your company performs a weekly full backup on Saturday. If a differential backup was run on Tuesday it would only backup data that had changed since Saturday.  Another differential backup run on Thursday would backup all the changed data from Saturday to Thursday.  This is fine, but differential backups can grow to be about the same size and take almost the same amount of time to complete as a full backup depending on the time period before the next full backup. That’s where incremental backups come in.

Incremental backups – Incremental backups backup the data that has been changed since the last backup whether that backup was a full, differential or incremental. So assuming you do a full backup on Saturday and an incremental backup Sunday through Friday. Sunday’s incremental will backup the changes since Saturday. Monday’s incremental will backup the data changed since Sunday.  Because in reality, most business have slowly changing data this makes incremental backups small and speedy allowing many backups per day if desired without impacting performance.  Incremental backups obviously give you more options when planning a backup strategy but they can take longer to restore from the backup has to be reconstructed from the last full backup to the last incremental backup.

  • It was a false sense of security because we couldn’t find a full backup. The incremental backups could never restore.
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