How does the garbage collector determine what to delete from /nix/store?

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1

Why does the garbage collector not delete the package in this scenario?

  1. Add a single package to configuration.nix
  2. Rebuild
  3. Delete that single added package
  4. Rebuild
  5. Run nix-collect-garbage -d as root

I expected that it would be deleted as it’s not part of the current generation.

2

The profile that’s set with configuration.nix is owned by root. You have to run nix-collect-garbage -d as root with sudo to get it to delete old system generations.

3

Seems like you don’t remove the generation. This single package is still linked to one of your old generations. Try to check with:
sudo nix-env -p /nix/var/nix/profiles/system --list-generations

If you remove all other generations with --delete-generations switch, you’ll probably see it’ll be the victim of new garbage collection.

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4

I see. So there are actually three “users” with profiles - me, root, and system? And I have to refer to /nix/var/nix/profiles/system to reference the system profile. That makes sense