In reply to @me:linj.tech
no, the old initrd just dies if one disk is missing. Because of that, I try this systemd one

  boot.initrd.luks.devices = {
    # TODO what if one disk is missing
    "nixos-1" = {
      allowDiscards = true;
      bypassWorkqueues = true;
      device = "/dev/disk/by-label/luks-1";
      # fallbackToPassword = true;
      # TODO x-initrd.attach
      # TODO x-systemd.device-timeout=
      crypttabExtraOpts = [ "nofail" "x-systemd.device-timeout=10" ];
    };
    "nixos-2" = {
      allowDiscards = true;
      bypassWorkqueues = true;
      device = "/dev/disk/by-label/luks-2";
      # fallbackToPassword = true;
      crypttabExtraOpts = [ "nofail" "x-systemd.device-timeout=10" ];
    };
  };

  fileSystems."/" =
    { device = "none";
      fsType = "tmpfs";
      options = [ "defaults" "size=500M" "mode=755" ];
    };

  # TODO what if one disk is missing
  fileSystems."/boot" =
    { device = "/dev/disk/by-label/efi-1";
      fsType = "vfat";
      options = [ "nofail" ];
    };

  # TODO what if one disk is missing
  fileSystems."/boot2" =
    { device = "/dev/disk/by-label/efi-2";
      fsType = "vfat";
      options = [ "nofail" ];
    };

  # TODO what if one disk is missing
  fileSystems."/nix" =
    { # device = "/dev/disk/by-label/nixos";
      device = "/dev/disk/by-uuid/f93cfbf1-e9b4-46ca-b7cb-6f3fb1554fbb";
      fsType = "btrfs";
      options = [ "degraded" "noatime" "ssd" "compress=zstd:1" "space_cache=v2" "subvol=@nix" ];
    };

  fileSystems."/var" =
    { # device = "/dev/disk/by-label/nixos";
      device = "/dev/disk/by-uuid/f93cfbf1-e9b4-46ca-b7cb-6f3fb1554fbb";
      fsType = "btrfs";
      options = [ "degraded" "noatime" "ssd" "compress=zstd:1" "space_cache=v2" "subvol=@var" ];
    };

  fileSystems."/home" =
    { # device = "/dev/disk/by-label/nixos";
      device = "/dev/disk/by-uuid/f93cfbf1-e9b4-46ca-b7cb-6f3fb1554fbb";
      fsType = "btrfs";
      options = [ "degraded" "noatime" "ssd" "compress=zstd:1" "space_cache=v2" "subvol=@home" ];
      neededForBoot = true;
    };

  fileSystems."/state" =
    { # device = "/dev/disk/by-label/nixos";
      device = "/dev/disk/by-uuid/f93cfbf1-e9b4-46ca-b7cb-6f3fb1554fbb";
      fsType = "btrfs";
      options = [ "degraded" "noatime" "ssd" "compress=zstd:1" "space_cache=v2" "subvol=@state" ];
      neededForBoot = true;
    };

In reply to @elvishjerricco:matrix.org
gotcha, so it's normal to just pick one of the devices in a btrfs raid array as the device?

In reply to @elvishjerricco:matrix.org
once you're in the emergency shell in the systemd stage 1, you can probably manually mount the degraded btrfs root and let it continue with systemctl default