One question I’m often asked by people who haven’t used differential levels within NetWorker is what is the purpose of the different levels? In particular, this tends to lead to if I’m going to implement a differential backup, what level should I choose?
While multiple differential levels grants considerable extra flexibility to NetWorker backups schedules, it can be confusing to the uninitiated. After all, if you’re currently only using fulls and incrementals, but want to insert a differential level, should you choose a level 1, level 2, level 3, etc.?
If you’re only using one differential level, it really doesn’t matter which level you use (though I have a suggestion); that is, all 9 differential levels interact with fulls and incrementals in exactly the same way when they are the only differential level used.
For this reason, if I’m only using one differential level in a schedule, I tend to use level 5. There’s a very simple reason for this – if at a later point I need to insert additional differential levels, then I can insert either lower or higher number levels around the 5 without having to change the level I’ve already been using, and with maximum flexibility as to the number of differential levels I can insert.
Now, how do they relate to one another?
- A differential level 1 will backup all changes since the last full.
- A differential level 2 will backup all the changes since the last full or the last differential level 1, whichever is most recent.
- A differential level 3 will backup all the changes since the last full or the last differential level 2, or the last differential level 1, whichever is most recent.
I.e., any differential level X will backup all files that have changed since either the last full or the last X-n level backup, whichever is more recent.
Now, in weekly schedules, there’s usually little point in using multiple differential levels. Where they come into play more usefully is in monthly or even more lengthy schedules such as quarterly backups. I’ve previously discussed using differential levels in monthly schedules here, and in my book I give a reasonably complete example of quarterly full backup schedules for small-change low-access servers.