NixOS Flakes transition: managing configuration drift during channel migration

Answer

NixOS does not automatically blend channel‑based packages with flake‑pinned inputs. When you invoke nixos-rebuild with the --flake flag, the build uses only the inputs locked in flake.lock; the traditional channel mechanism is ignored. Without --flake, the system continues to rely on nix-channel entries. Therefore, during a phased migration you must decide which mode each rebuild uses, otherwise version conflicts can cause silent drift.

Confirmed facts

• Flakes require experimental-features = nix-command flakes in nix.conf.
• The --flake flag makes Nix ignore $NIX_PATH and channel‑derived nixpkgs.
• If a module or overlay references a channel‑specific revision that is not present in the flake inputs, that reference will resolve to whatever the channel provides (or fail), leading to drift.

Steps for a safe migration

1. Audit current channel usage: run nix-channel list and check /etc/nixos/configuration.nix for imports like or overlays that depend on a specific channel revision.
2. Create a flake:

   # flake.nix
   {
     description = "NixOS system migrated to flakes";
     inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
     outputs = { self, nixpkgs }:
       let
         system = "x86_64-linux";
         pkgs = import nixpkgs { inherit system; };
       in
       {
         nixosConfigurations.myhost = nixpkgs.lib.nixosSystem {
           inherit system;
           modules = [ ./configuration.nix ];
         };
       };
   }
   

3. Lock inputs: nix flake update (creates/updates flake.lock).
4. Test the build: nix build .#nixosConfigurations.myhost.config.system.build.toplevel. Verify no errors.
5. Backup the current generation: sudo nix-store --realise $(readlink -f /run/current-system) (store path) or simply note the current generation number with sudo nix-env --list-generations -p /nix/var/nix/profiles/system.
6. Switch to the flake: sudo nixos-rebuild switch --flake .#myhost.
7. Verify no drift:
   • Compare generations: sudo nix-env --list-generations -p /nix/var/nix/profiles/system should show a new generation.
   • Run nix store diff-closures (or use nix diff-closures) to ensure the closure of the new system matches the expected set of packages.
   • Check nixos-version matches the revision recorded in flake.lock.
8. Rollback if needed: sudo nixos-rebuild switch --rollback returns to the last working channel‑based generation.

When to ask for a missing detail

If your configuration.nix contains overlays, module imports, or pkgs lookups that depend on a channel‑specific nixpkgs revision (e.g., import {} with a pinned channel), you must replicate that pinning in the flake inputs. Without knowing whether such channel‑specific dependencies exist, the recommendation to keep the flake inputs unchanged may be insufficient. Please confirm whether any part of your configuration relies on a specific channel revision.