In the 90's I remember using various backup tools that had their own custom format. I decided that I wanted my backup to be just wanted files on a normal filesystem without being stored in some kind of special format. I realize that this means compression and block level deduplication is harder but those are tradeoffs that I can accept.
I've been using rsnapshot (or it's predecessor script) for 20 years now. It's a wrapper around rsync that gives you snapshots that save space by using hard links. I can compare versions with diff. I can restore files with cp -a
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