The roots of the garbage collector are all store paths to which there are symlinks in the directory prefix/nix/var/nix/gcroots. For instance, the following command makes the path /nix/store/d718ef...-foo a root of the collector: $ ln -s /nix/store/d718ef...-foo /nix/var/nix/gcroots/bar That is, after this command, the garbage collector will not remove /nix/store/d718ef...-foo or any of its dependencies. Subdirectories of prefix/nix/var/nix/gcroots are also searched for symlinks. Symlinks to non-store paths are followed and searched for roots, but symlinks to non-store paths inside the paths reached in that way are not followed to prevent infinite recursion.