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Some backup software vendors propose making weekly "full backups" in addition to daily "incremental backups". Point in case: http://novabackup.novastor.com/blog/differential-incremental-backup/

I have the following problem with this:

  • If you do trust your incremental backups you should not need "full" backups
  • If you don't trust your incremental backups then you should take "full" backups daily and don't use incremental backups at all

Why would you take both incremental and full backups as suggested in the blog post linked above?

  • I'm voting to close this question as off-topic because poster doesn't know the basics and has clearly done no homework of their own. – Chopper3 Apr 4 '17 at 11:09
  • I did research the topic but only found answers as the one i cited. I did search serverfault before posting. Yes, I'm not knowledgable about the issue but I don't think it's a stupid question and the answers are really good. – gardarh Apr 4 '17 at 11:40
  • This could better fit in Super User as it certainly helps home users to consider between these options. Professionals should already know as this isn't something specific but part of fundamental skills. – Esa Jokinen Apr 4 '17 at 15:09
  • Hmmm, I never really researched the difference between serverfault and superuser (the two are easily confused) but I see your point. – gardarh Apr 5 '17 at 15:57
5

Well, the reason for that is not trust in the backup. It's the amount of work for a restore (among many other considerations that go in a backup scheme). If you have only one full and 300 incremental backups, you need to restore first the full and then every single incremental backup to get to the actual data.

If you have weekly full + incremental, you need to restore at max 7 backups.

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The need of full backups is more obvious when more time goes by. When recovering from the incremental backups you go through all the changes after last full backup in order to recover the current state of every file.

How often you should take full backups depends on how long you need to keep incremental data i.e. how distant past versions you may need to recover. You can make calculations on how much data your backup will have based on variables:

  • size of the one full backup
  • average changes per daily incremental backup
  • days between full backups
  • total days you keep backups
  • do you need to store incremental data only after last full backup or all of them

For example if you have 700 GB of total data with average 3 GB of daily changes and you keep backups for half a year, the total amount of data in your backups:

  • Weekly full backup: 700 GB * 26 + 3 GB * 6 = 18 TB of data to backup
  • Monthly full backups: 700 GB * 6 + 3 GB * 30 = 4,3 TB of data to backup
  • Quarterly full backups: 700 GB * 2 + 3 GB * 90 = 1,7 TB of data to backup

And if you only take one backup and then keep taking incremental backups for 4 years, you will end up with original 700 GB full backup and 3 GB * 365 * 4 = 4,3 TB of incremental backups you need to go through, when you perform a full recovery.

There isn't one correct solution for time between full backups, but with these variables you can count what you value the most in your situation.

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It isn't just about trusting the backup application either, there are or can be a few things that can wrong, even if the application reports everything is fine.

What happens if the application is 100% perfect but you lose a backup, the backup media gets damaged, or fails to upload (cloud based backups) correctly?

What about speed of restore? I'd rather restore a single weekly backup + a maximum of a weeks worth of incremental backups, than. A single full + a year or mores worth of incremental backups. Imagine looking a single file and someone tells you it was changed between two dates. The dates are 4 weeks apart. You can search 4 weekly backups or 28 (or more) incremental backups?

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