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

NixOS

NixOS itself obviously needs /nix to be persistent, so the system environment can be linked from the nix store during activation.

Moreover, /boot should also be persistent, as it contains the kernel, initrd, and the command-line determining the system environment's path in the nix store.

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).