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

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).