diff --git a/nixos/lib/utils.nix b/nixos/lib/utils.nix
index c9dfdbed99a3..1a51b149e56e 100644
--- a/nixos/lib/utils.nix
+++ b/nixos/lib/utils.nix
@@ -14,8 +14,30 @@ rec {
   fsNeededForBoot = fs: fs.neededForBoot || elem fs.mountPoint pathsNeededForBoot;
 
   # Check whenever `b` depends on `a` as a fileSystem
-  fsBefore = a: b: a.mountPoint == b.device
-                || hasPrefix "${a.mountPoint}${optionalString (!(hasSuffix "/" a.mountPoint)) "/"}" b.mountPoint;
+  fsBefore = a: b:
+    let
+      # normalisePath adds a slash at the end of the path if it didn't already
+      # have one.
+      #
+      # The reason slashes are added at the end of each path is to prevent `b`
+      # from accidentally depending on `a` in cases like
+      #    a = { mountPoint = "/aaa"; ... }
+      #    b = { device     = "/aaaa"; ... }
+      # Here a.mountPoint *is* a prefix of b.device even though a.mountPoint is
+      # *not* a parent of b.device. If we add a slash at the end of each string,
+      # though, this is not a problem: "/aaa/" is not a prefix of "/aaaa/".
+      normalisePath = path: "${path}${optionalString (!(hasSuffix "/" path)) "/"}";
+      normalise = mount: mount // { device = normalisePath mount.device;
+                                    mountPoint = normalisePath mount.mountPoint;
+                                    depends = map normalisePath mount.depends;
+                                  };
+
+      a' = normalise a;
+      b' = normalise b;
+
+    in hasPrefix a'.mountPoint b'.device
+    || hasPrefix a'.mountPoint b'.mountPoint
+    || any (hasPrefix a'.mountPoint) b'.depends;
 
   # Escape a path according to the systemd rules, e.g. /dev/xyzzy
   # becomes dev-xyzzy.  FIXME: slow.
diff --git a/nixos/modules/tasks/filesystems.nix b/nixos/modules/tasks/filesystems.nix
index 065d6cc95d18..2949c82df8fe 100644
--- a/nixos/modules/tasks/filesystems.nix
+++ b/nixos/modules/tasks/filesystems.nix
@@ -24,13 +24,15 @@ let
 
   specialFSTypes = [ "proc" "sysfs" "tmpfs" "ramfs" "devtmpfs" "devpts" ];
 
+  nonEmptyWithoutTrailingSlash = addCheckDesc "non-empty without trailing slash" types.str
+    (s: isNonEmpty s && (builtins.match ".+/" s) == null);
+
   coreFileSystemOpts = { name, config, ... }: {
 
     options = {
       mountPoint = mkOption {
         example = "/mnt/usb";
-        type = addCheckDesc "non-empty without trailing slash" types.str
-          (s: isNonEmpty s && (builtins.match ".+/" s) == null);
+        type = nonEmptyWithoutTrailingSlash;
         description = "Location of the mounted the file system.";
       };
 
@@ -55,6 +57,20 @@ let
         type = types.listOf nonEmptyStr;
       };
 
+      depends = mkOption {
+        default = [ ];
+        example = [ "/persist" ];
+        type = types.listOf nonEmptyWithoutTrailingSlash;
+        description = ''
+          List of paths that should be mounted before this one. This filesystem's
+          <option>device</option> and <option>mountPoint</option> are always
+          checked and do not need to be included explicitly. If a path is added
+          to this list, any other filesystem whose mount point is a parent of
+          the path will be mounted before this filesystem. The paths do not need
+          to actually be the <option>mountPoint</option> of some other filesystem.
+        '';
+      };
+
     };
 
     config = {