In reply to @elvishjerricco:matrix.org
even if you're not using udev/systemd, the btrfs tools by default don't let you mount degraded

I think I see how this works:

https://github.com/systemd/systemd/blob/main/rules.d/64-btrfs.rules.in

Basically there's a udev builtin btrfs ready that checks if a device is ready to be used as a btrfs FS; i.e. if all its partner disks are present. If not, it sets SYSTEMD_READY=0 so that it doesn't activate the .device unit. Once one of them says its ready, it allows SYSTEMD_READY to remain 1, then triggers the others to check again. This will (presumably) cause all the others to check and set SYSTEMD_READY=1. Now if any of those devices is the device unit for your mount unit, the mount unit can now activate

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;
    };