build1.json

{
  "customdata": {
    "cpr_storage": {
      "disks": [
        {
          "device": "/dev/sdd",
          "partitions": [
            {
              "label": "BIOS",
              "number": 1,
              "size": "512M"
            },
            {
              "label": "SWAP",
              "number": 2,
              "size": "4G"
            },
            {
              "label": "POOL",
              "number": 3,
              "size": 0
            }
          ]
        }
      ],
      "filesystems": [
        {
          "mount": {
            "create": {
              "options": [
                "32",
                "-n",
                "EFI"
              ]
            },
            "device": "/dev/sdd1",
            "format": "vfat",
            "point": "/boot"
          }
        },
        {
          "mount": {
            "create": {
              "options": [
                "-L",
                "SWAP"
              ]
            },
            "device": "/dev/sdd2",
            "format": "swap",
            "point": "none"
          }
        }
      ]
    },
    "cpr_zfs": {
      "datasets": {
        "tank/home": {
          "properties": {
            "mountpoint": "legacy"
          }
        },
        "tank/nix": {
          "properties": {
            "com.sun:auto-snapshot": "false",
            "mountpoint": "legacy"
          }
        },
        "tank/root": {
          "properties": {
            "mountpoint": "legacy"
          }
        },
        "tank/var": {
          "properties": {
            "mountpoint": "legacy"
          }
        }
      },
      "mounts": [
        {
          "dataset": "tank/root",
          "point": "/"
        },
        {
          "dataset": "tank/nix",
          "point": "/nix"
        },
        {
          "dataset": "tank/var",
          "point": "/var"
        },
        {
          "dataset": "tank/home",
          "point": "/home"
        }
      ],
      "pools": {
        "tank": {
          "pool_properties": {
            "ashift": "12"
          },
          "vdevs": [
            {
              "disk": [
                "/dev/sda",
                "/dev/sdb",
                "/dev/sdc",
                "/dev/sdd3"
              ]
            }
          ]
        }
      }
    }
  }
}

default.nix

with import <nixpkgs> {};
let
  createSystem = { disks, vdevs }: {
    customdata = {
      cpr_storage = {
        inherit disks;
        filesystems = [
          {
            mount = {
              device = "/dev/sdd1";
              format = "vfat";
              point = "/boot";
              create.options = [ "32" "-n" "EFI" ];
            };
          }
          {
            mount = {
              device = "/dev/sdd2";
              format = "swap";
              point = "none";
              create.options = [ "-L" "SWAP" ];
            };
          }
        ];
      };
      cpr_zfs = {
        pools = {
          tank = {
            pool_properties = {
              ashift = "12"; # 4kbyte block size
            };
            inherit vdevs;
          };
        };
        datasets = let
          common = {
            properties.mountpoint = "legacy";
          };
        in {
          "tank/root" = common;
          "tank/nix" = lib.recursiveUpdate common { properties."com.sun:auto-snapshot" = "false"; };
          "tank/home" = common;
          "tank/var" = common;
        };
        mounts = let
          fn = prefix: { dataset = "tank/${prefix}"; point = "/${prefix}"; };
        in [
          { dataset = "tank/root"; point = "/"; }
          (fn "nix")
          (fn "var")
          (fn "home")
        ];
      };
    };
  };
  mkPrimaryDisk = { efi ? true, legacy ? false, swapSize ? "4G", device }: {
    device = "/dev/${device}";
    partitions = [
      { label = "BIOS"; number = 1; size = "512M"; }
      { label = "SWAP"; number = 2; size = swapSize; }
      { label = "POOL"; number = 3; size = 0; }
    ];
  };
  hydra-master = createSystem {
    disks = [
      (mkPrimaryDisk { device = "sdd"; })
    ];
    vdevs = [
      {
        raidz1 = [ "/dev/sda" "/dev/sdb" "/dev/sdc" "/dev/sdd3" ];
      }
    ];
  };
  hydra-build-machine = createSystem {
    disks = [
      (mkPrimaryDisk { device = "sdd"; })
    ];
    vdevs = [
      {
        disk = [ "/dev/sda" "/dev/sdb" "/dev/sdc" "/dev/sdd3" ];
      }
    ];
  };
  prettyJSON = object: builtins.toFile "temp.json" (builtins.toJSON object);
  machines = {
    inherit hydra-master;
    build1 = hydra-build-machine;
  };
  generateOutput = machines: let
    fn = name: obj: "jq . -S < ${prettyJSON obj} > ${name}.json";
    scripts = lib.concatStringsSep "\n" (lib.mapAttrsFlatten fn machines);
  in runCommand "output" { buildInputs = [ jq ]; preferLocalBuild = true; } ''
    mkdir $out
    cd $out
    ${scripts}
  '';
in
  generateOutput machines

hydra-master.json

{
  "customdata": {
    "cpr_storage": {
      "disks": [
        {
          "device": "/dev/sdd",
          "partitions": [
            {
              "label": "BIOS",
              "number": 1,
              "size": "512M"
            },
            {
              "label": "SWAP",
              "number": 2,
              "size": "4G"
            },
            {
              "label": "POOL",
              "number": 3,
              "size": 0
            }
          ]
        }
      ],
      "filesystems": [
        {
          "mount": {
            "create": {
              "options": [
                "32",
                "-n",
                "EFI"
              ]
            },
            "device": "/dev/sdd1",
            "format": "vfat",
            "point": "/boot"
          }
        },
        {
          "mount": {
            "create": {
              "options": [
                "-L",
                "SWAP"
              ]
            },
            "device": "/dev/sdd2",
            "format": "swap",
            "point": "none"
          }
        }
      ]
    },
    "cpr_zfs": {
      "datasets": {
        "tank/home": {
          "properties": {
            "mountpoint": "legacy"
          }
        },
        "tank/nix": {
          "properties": {
            "com.sun:auto-snapshot": "false",
            "mountpoint": "legacy"
          }
        },
        "tank/root": {
          "properties": {
            "mountpoint": "legacy"
          }
        },
        "tank/var": {
          "properties": {
            "mountpoint": "legacy"
          }
        }
      },
      "mounts": [
        {
          "dataset": "tank/root",
          "point": "/"
        },
        {
          "dataset": "tank/nix",
          "point": "/nix"
        },
        {
          "dataset": "tank/var",
          "point": "/var"
        },
        {
          "dataset": "tank/home",
          "point": "/home"
        }
      ],
      "pools": {
        "tank": {
          "pool_properties": {
            "ashift": "12"
          },
          "vdevs": [
            {
              "raidz1": [
                "/dev/sda",
                "/dev/sdb",
                "/dev/sdc",
                "/dev/sdd3"
              ]
            }
          ]
        }
      }
    }
  }
}