Fix everything.

system/devices/boot/services/default.nix

@@ -0,0 +1,6 @@
+{ ... }:
+{
+  imports = [ ./root-reset ];
+
+  boot.initrd.systemd.enable = true;
+}

system/devices/boot/services/root-reset/default.nix

@@ -0,0 +1,31 @@
+{ config, ... }:
+{
+  boot.initrd.systemd.services.root-reset = {
+    enable = config.environment.persistence."/persist".enable;
+    description = "Create new and snapshot previous root";
+    wantedBy = [ "initrd.target" ];
+    before = [ "sysroot.mount" ];
+    after = [ "initrd-root-device.target" ];
+    unitConfig.DefaultDependencies = "no";
+    serviceConfig.Type = "oneshot";
+    script = ''
+      mkdir -p /mnt
+      mount -t btrfs /dev/${config.networking.hostName}/root /mnt
+
+      if [[ -e /mnt/prev ]]; then
+        btrfs subvolume delete /mnt/prev
+      fi
+
+      btrfs subvolume snapshot /mnt/root /mnt/prev
+
+      btrfs subvolume list -o /mnt/root | cut -f9 -d' ' | while read subvolume; do
+        btrfs subvolume delete "/mnt/$subvolume"
+      done
+
+      btrfs subvolume delete /mnt/root
+      btrfs subvolume create /mnt/root
+
+      umount /mnt
+    '';
+  };
+}