Switching between `nix build` builder profiles

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1

Remote builders can save build time, especially when they are use from multiple machines and also serve as a cache.

Remote builders can be specified in /etc/nix/nix.conf via configuration.nix, e.g. with

  nix = {
    settings = {
      max-jobs = lib.mkDefault 4; # this machine is a laptop
    };
    distributedBuilds = true;
    buildMachines = [{
      hostName = "builder.example.dk";
      system = "x86_64-linux";
      protocol = "ssh"; # ssh-ng seems slower
      speedFactor = 5; # this machine is fast
      maxJobs = 4;
      supportedFeatures = [ "nixos-test" "benchmark" "big-parallel" "kvm" ];
      mandatoryFeatures = [ ];
    }, {
      # another remote builder
    }];
  };

Fairly regularly, I’d like to not use the remote builder. When I have a bad network connection or when the remote builder is overloaded. Since the remote builders are configured in /etc/nix/nix.conf via configuration.nix changing that setting requires a cumbersome nixos-rebuild.

Is there more convenient / faster way to switch?

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2

Try nix build --builders "" ...

3

That works great!

Ideally, it’s be quick to switch between remote build servers too, but that might be handled by aliases or scripts.

4

You can provide the same info to the --builders option as you do in configuration.nix, see Remote Builds - Nix Reference Manual. However it is easy to forget to for example provide the maxJobs parameter when using --builders, so if you do something like --builders ssh://builder.example.dk you have, perhaps unknowingly, restricted the remote builder to only one job at a time.

It would be nice if you could add aliases for the machines together with their parameters, and then simply select which of those aliases to use with the --builders option.

5

Yes, selecting between profiles with --builders would be great.

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6

The problem is that --builders currently is used either for pointing Nix to a file containing builder machines (using the @/path/to/builders syntax), or directly define the builder machines. Perhaps a new option is needed to define the “available builders”, and then some syntax for --builders to select builders from that list.

7

The documentation (man nix) just says:

  · --builders value
    
    Set the builders setting.

I had no idea it can be used to point to a file with builder definitions. That is good way to switch profiles too.

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