tl;dr: Picking what you are going to back up helps (i) keep the backup space usage minimal (ii) helps to inform choice of backup program
You could make the argument that really, what you’re going to back up is part of your requirements gathering. Frequently-changing data (eg documents) is different from a snapshot of a Windows installation is different from an archive of the family photos.
My my case, I want to back up my home directory, which is a mix of things:
- documents of all sorts
- code (some mine, some open source tools)
- application configuration data
- browser history etc
- miscellaneous downloads
It totals less than 20Gb, most of which is split between downloads, browser and code (around 3:1:1, according to
ncdu). Some things like documents, code and browser data will change semi-frequently and old versions are useful; others will stay relatively static and version history is not so important (like downloads).
Some downloads were for a one-off specific purpose and removed. It would be possible to pare down further by removing some downloads and some code — wine is the largest directory in
~/code/, and I don’t remember the last time I used it — but it’s not enough that I feel it’s a priority to do.
Is there anything in this set of data that doesn’t need kept? Frequently-changing-but-low-utility files like browser cache would be worth excluding as they will cause the (incremental) backups to grow in size. Incidentally, cache was the next largest item in the ratio above!
Some of the files will change relatively frequently, and I’d like to keep the history of them. I have decided that I want to keep my entire home directory, minus browser cache. This help to inform me what things I need my backup program to do, and what to do with it when I decide.