> Ok, so like filesystem snapshots.
Sort of, but only sort of. The filesystem is actually (mostly) unchanged for the backups. You have the /nix partition that can only really be touched by the nix program itself, and you have your root partition built based on the manifest defined in your configuration file. As such, every time you want to change anything about the system, you "rebuild" it, it stores a "copy" of the old root system, but since it's a cool lazy functional thing, it really only stores the diff's from the configuration, so they aren't terribly expensive, so it's not weird for me to have literally hundreds of generations stored on my laptop. Old stuff in the /nix partition isn't deleted until it's garbage collected, and a lot of stuff can't be garbage collected until the generation attached to it is removed.
So it's like a filesystem snapshot, but only the system files itself, the actual stuff on the hard drive actually doesn't get touched.
> However, since LiveCD/Flash images became a thing, I stopped worrying about the OS at all, just keep a recent backup of my /data partition.
I actually kind of do that with NixOS too!
My root partition is a tmpfs mount, meaning that every reboot is effectively a fresh root [1]. I have persistent volumes for my home directory and a few other things. To quote a friend, it's like a "new car smell on every reboot".
[1] Followed this tutorial: https://elis.nu/blog/2020/05/nixos-tmpfs-as-root/