#+title: ZFS Support
#+setupfile: ../headers

* ZFS Support
Enabling ZFS is quite straightforward on my machines. In my module
=nixos.zfs=, I expose two options: which ZFS pools to import, and
whether to force-import the root pool. But first, here’s the skeleton
of my module.

#+begin_src nix :tangle yes
{
  flake.modules.nixos.zfs = {
    lib,
    config,
    ...
  }:
    with lib; let
      cfg = config.mySystem.boot;
    in {
      <<options>>
      <<enable>>
      <<pools>>
      <<force-import-root>>
    };
}
#+end_src

The main option is the list of pools, which I pass directly to the
standard NixOS configuration.
#+name: options
#+begin_src nix
options.mySystem.boot.zfs = {
  pools = mkOption {
    type = types.listOf types.str;
    default = [];
  };
  forceImportRoot = mkEnableOption "Force-import the ZFS root pool at boot, even if it looks already imported elsewhere";
};
#+end_src

Now, I can enable ZFS on the system importing the current module.
#+name: enable
#+begin_src nix
config.boot.supportedFilesystems = ["zfs"];
#+end_src

And lastly, passing the list of pools to the standard NixOS option is
quite simple.
#+name: pools
#+begin_src nix
config.boot.zfs.extraPools = cfg.zfs.pools;
#+end_src

Force-importing the root pool can paper over a stale or mismatched
host ID, but it also risks mounting a pool that’s already imported
elsewhere and corrupting it, so NixOS is moving to disable it by
default. I’d rather keep that safety and only turn it back on for the
rare host (or the rare boot) that actually needs it.
#+name: force-import-root
#+begin_src nix
config.boot.zfs.forceImportRoot = cfg.zfs.forceImportRoot;
#+end_src