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Should I use differential or incremental backups?

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I’m going to talk about whether one should use differential or incremental backups?
This is a question many people frequently ask. So what are the advantages and disadvantages of each?

Assumptions
For the purpose of simplicity, we'll assume you are backing up files based on the archive bit. This is a marker on each file, which gets "turned on" when a file is created or modified. In Windows you can see it by viewing a file's advanced properties (It's called File is ready for archiving).

When you perform a full backup, every file included in your selection lists (apart from those excluded by active file exclusion) are backed up, and the archive bit is "turned off" on each file.

Incremental Backup
When you perform an incremental backup, every file included in your selection lists (apart from those excluded by an active file exclusion) that has the archive bit "turned on" is backed up, and the then the archive bit is "turned off" for each file.

Differential Backup
When you perform a differential backup, every file included in your selection lists (apart from those excluded by an active file exclusion) that has the archive bit "turned on" is backed up, but the archive bit is not changed at all.

This means that instead of performing full backups each night, you can avoid backing up the data that has not changed. This saves time and storage space.

Three Typical Backup Examples:
Company A, Company B and Company C each have 100GB of data to backup, and each day 1 GB of that data changes.

Company A performs a full backup each night.
Company B performs a full backup on each Monday night and an incremental backup each night.
Company C performs a full backup on each Monday night and a differential backup each night.

Company A will perform a full backup 100GB of data each night. At the end of the week they have backed up 700GB of data.

Company B will backup 100GB of data each Monday night (full backup), and an incremental backup of 1GB of data (the changed files) every other night.  By Friday, 105GB of data has been backed up.

Company C will do a full backup 100GB of data each Monday night and a differential backup of 1GB of data on the Tuesday night. On the Wednesday night, the differential backup will be between 1 and 2 GB of data, depending upon whether the files changed on Wednesday were the same files changed on Tuesday. Continuing the differential backups through Friday will result in having backed up between 1 and 4 GB of data.

Backup Comparison
So Companies B and C will spend far less time (and resources) backing up than Company A. Whether Company B spends much less time backing up than Company C depends upon their data usage.

The Restore Process?
Let's assume Companies A, B and C all have a catastrophic failure on a Monday morning.

Company A can restore from the Friday night media. Nothing else is needed. If Friday's tape won't restore for some reason, they can try Thursday's tape.

Company B will need to restore from the previous Monday night backup. Then the Tuesday, Wednesday, Thursday and Friday backups will each need to be restored (in order).

Company C will need to restore from the previous Monday night backup, and then restore the Friday night backup. It will not need Tuesday, Wednesday, and Thursday backups.

Comparison Of The Restores
Obviously Company A's recovery will be quickest, Company C's will be the second fastest, and Company B's will take the longest.

Potential Problems
There remains a potential problem, however for Companies B and C. If a file has been deleted, this information does not get stored on the backup tape (unless you use the Advanced Disk-based Backup Option, but for the purposes of this article we will ignore that topic).

If a file existed on the Monday night for Company C, but was deleted during the week, it will be restored from the Monday night backup, and when the Friday backup has been restored, the restored file will still exist, even though it wasn't there when the Friday night backup was taken.

Similarly, for Company B, If a file existed on any weeknight for Company B, but was deleted during the week, it will be restored from the Monday night backup, or the backup of the day it was created, and when the Friday backup has been restored, the restored file will still exist, even though it wasn't there when the Friday night backup was taken.

As a result, Company C may have some files around which should not exist.  Company B may have many files which should not exist.

If you have an application which relies on the presence or absence of files, this may cause issues for Companies B and C.

Conclusion
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Comment
Author:jakirkhan
1 Comment
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Expert Comment

by:rdivilbiss
I note you have written this article in the "Windows XP Operating System" Zone.  The article certainly applies to many operating systems, not juse Windows XP.  I suggest the article be moved the the Backup / Restore zone.

Regards,
Rod
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