feat(nixos): span zeus root across nvme disks

configs/nixos/hosts/zeus/disk.nix

@@ -1,10 +1,10 @@
 { ... }:
-# zeus's disk layout for disko: one NVMe disk, GPT, with an EFI system
-# partition and a LUKS container holding btrfs subvolumes.
+# zeus's disk layout for disko: two NVMe disks with one EFI system partition
+# and two LUKS containers joined into one btrfs filesystem.
 # No swap partition, since swap is zram.
 # disko derives `fileSystems` and `boot.initrd.luks.devices` from this.
 {
-  disko.devices.disk.main = {
+  disko.devices.disk.primary = {
     type = "disk";
     device = "/dev/nvme0n1";
     content = {
@@ -24,11 +24,34 @@
           size = "100%";
           content = {
             type = "luks";
-            name = "cryptroot";
+            name = "cryptroot0";
+            settings.allowDiscards = true;
+          };
+        };
+      };
+    };
+  };
+
+  disko.devices.disk.secondary = {
+    type = "disk";
+    device = "/dev/nvme1n1";
+    content = {
+      type = "gpt";
+      partitions = {
+        luks = {
+          size = "100%";
+          content = {
+            type = "luks";
+            name = "cryptroot1";
             settings.allowDiscards = true;
             content = {
               type = "btrfs";
-              extraArgs = [ "-f" ];
+              extraArgs = [
+                "-f"
+                "-d single"
+                "-m raid1"
+                "/dev/mapper/cryptroot0"
+              ];
               subvolumes = {
                 "@root" = {
                   mountpoint = "/";