nixpkgs/nixos/doc/manual/administration/nixos-state.section.md

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NixOS

/nix

NixOS needs the entirety of /nix to be persistent, as it includes:

  • /nix/store, which contains all the system's executables, libraries, and supporting data;
  • /nix/var/nix, which contains:
    • the Nix daemon's database;
    • roots whose transitive closure is preserved when garbage-collecting the Nix store;
    • system-wide and per-user profiles.

/boot

/boot should also be persistent, as it contains:

  • the kernel and initrd which the bootloader loads,
  • the bootloader's configuration, including the kernel's command-line which determines the store path to use as system environment.

Users and groups

  • /var/lib/nixos should persist: it holds state needed to generate stable uids and gids for declaratively-managed users and groups, etc.
  • users.mutableUsers should be false, or the following files under /etc should all persist:
    • {manpage}passwd(5) and {manpage}group(5),
    • {manpage}shadow(5) and {manpage}gshadow(5),
    • {manpage}subuid(5) and {manpage}subgid(5).