diff --git a/common/prefs.cc b/common/prefs.cc
index 6a33037..6db01b7 100644
--- a/common/prefs.cc
+++ b/common/prefs.cc
@@ -28,11 +28,27 @@
 #include "update_engine/common/utils.h"
 
 using std::string;
+using std::vector;
 
 namespace chromeos_update_engine {
 
 const char kKeySeparator = '/';
 
+namespace {
+
+void DeleteEmptyDirectories(const base::FilePath& path) {
+  base::FileEnumerator path_enum(
+      path, false /* recursive */, base::FileEnumerator::DIRECTORIES);
+  for (base::FilePath dir_path = path_enum.Next(); !dir_path.empty();
+       dir_path = path_enum.Next()) {
+    DeleteEmptyDirectories(dir_path);
+    if (base::IsDirectoryEmpty(dir_path))
+      base::DeleteFile(dir_path, false);
+  }
+}
+
+}  // namespace
+
 bool PrefsBase::GetString(const string& key, string* value) const {
   return storage_->GetKey(key, value);
 }
@@ -108,11 +124,8 @@
     observers_for_key.erase(observer_it);
 }
 
-string PrefsInterface::CreateSubKey(const string& name_space,
-                                    const string& sub_pref,
-                                    const string& key) {
-  return base::JoinString({name_space, sub_pref, key},
-                          string(1, kKeySeparator));
+string PrefsInterface::CreateSubKey(const vector<string>& ns_and_key) {
+  return base::JoinString(ns_and_key, string(1, kKeySeparator));
 }
 
 // Prefs
@@ -124,23 +137,7 @@
 bool Prefs::FileStorage::Init(const base::FilePath& prefs_dir) {
   prefs_dir_ = prefs_dir;
   // Delete empty directories. Ignore errors when deleting empty directories.
-  base::FileEnumerator namespace_enum(
-      prefs_dir_, false /* recursive */, base::FileEnumerator::DIRECTORIES);
-  for (base::FilePath namespace_path = namespace_enum.Next();
-       !namespace_path.empty();
-       namespace_path = namespace_enum.Next()) {
-    base::FileEnumerator sub_pref_enum(namespace_path,
-                                       false /* recursive */,
-                                       base::FileEnumerator::DIRECTORIES);
-    for (base::FilePath sub_pref_path = sub_pref_enum.Next();
-         !sub_pref_path.empty();
-         sub_pref_path = sub_pref_enum.Next()) {
-      if (base::IsDirectoryEmpty(sub_pref_path))
-        base::DeleteFile(sub_pref_path, false);
-    }
-    if (base::IsDirectoryEmpty(namespace_path))
-      base::DeleteFile(namespace_path, false);
-  }
+  DeleteEmptyDirectories(prefs_dir_);
   return true;
 }
 
diff --git a/common/prefs_interface.h b/common/prefs_interface.h
index b559697..3aad480 100644
--- a/common/prefs_interface.h
+++ b/common/prefs_interface.h
@@ -20,6 +20,7 @@
 #include <stdint.h>
 
 #include <string>
+#include <vector>
 
 namespace chromeos_update_engine {
 
@@ -80,9 +81,7 @@
   virtual bool Delete(const std::string& key) = 0;
 
   // Creates a key which is part of a sub preference.
-  static std::string CreateSubKey(const std::string& name_space,
-                                  const std::string& sub_pref,
-                                  const std::string& key);
+  static std::string CreateSubKey(const std::vector<std::string>& ns_with_key);
 
   // Add an observer to watch whenever the given |key| is modified. The
   // OnPrefSet() and OnPrefDelete() methods will be called whenever any of the
diff --git a/common/prefs_unittest.cc b/common/prefs_unittest.cc
index f226949..24a62b5 100644
--- a/common/prefs_unittest.cc
+++ b/common/prefs_unittest.cc
@@ -62,17 +62,27 @@
 
 TEST(Prefs, Init) {
   Prefs prefs;
-  const string name_space = "ns";
+  const string ns1 = "ns1";
+  const string ns2A = "ns2A";
+  const string ns2B = "ns2B";
   const string sub_pref = "sp";
 
   base::ScopedTempDir temp_dir;
   ASSERT_TRUE(temp_dir.CreateUniqueTempDir());
-  base::FilePath namespace_path = temp_dir.GetPath().Append(name_space);
+  auto ns1_path = temp_dir.GetPath().Append(ns1);
+  auto ns2A_path = ns1_path.Append(ns2A);
+  auto ns2B_path = ns1_path.Append(ns2B);
+  auto sub_pref_path = ns2A_path.Append(sub_pref);
 
-  EXPECT_TRUE(base::CreateDirectory(namespace_path.Append(sub_pref)));
-  EXPECT_TRUE(base::PathExists(namespace_path.Append(sub_pref)));
+  EXPECT_TRUE(base::CreateDirectory(ns2B_path));
+  EXPECT_TRUE(base::PathExists(ns2B_path));
+
+  EXPECT_TRUE(base::CreateDirectory(sub_pref_path));
+  EXPECT_TRUE(base::PathExists(sub_pref_path));
+
+  EXPECT_TRUE(base::PathExists(ns1_path));
   ASSERT_TRUE(prefs.Init(temp_dir.GetPath()));
-  EXPECT_FALSE(base::PathExists(namespace_path));
+  EXPECT_FALSE(base::PathExists(ns1_path));
 }
 
 TEST_F(PrefsTest, GetFileNameForKey) {
@@ -99,9 +109,9 @@
   const string sub_pref2 = "sp2";
   const string sub_key = "sk";
 
-  EXPECT_EQ(PrefsInterface::CreateSubKey(name_space, sub_pref1, sub_key),
+  EXPECT_EQ(PrefsInterface::CreateSubKey({name_space, sub_pref1, sub_key}),
             "ns/sp1/sk");
-  EXPECT_EQ(PrefsInterface::CreateSubKey(name_space, sub_pref2, sub_key),
+  EXPECT_EQ(PrefsInterface::CreateSubKey({name_space, sub_pref2, sub_key}),
             "ns/sp2/sk");
 }
 
@@ -312,8 +322,8 @@
   const string sub_pref = "sp";
   const string sub_key1 = "sk1";
   const string sub_key2 = "sk2";
-  auto key1 = prefs_.CreateSubKey(name_space, sub_pref, sub_key1);
-  auto key2 = prefs_.CreateSubKey(name_space, sub_pref, sub_key2);
+  auto key1 = prefs_.CreateSubKey({name_space, sub_pref, sub_key1});
+  auto key2 = prefs_.CreateSubKey({name_space, sub_pref, sub_key2});
   base::FilePath sub_pref_path = prefs_dir_.Append(name_space).Append(sub_pref);
 
   ASSERT_TRUE(prefs_.SetInt64(key1, 0));
@@ -350,7 +360,7 @@
   prefs_.Delete(kKey);
   testing::Mock::VerifyAndClearExpectations(&mock_obserser);
 
-  auto key1 = prefs_.CreateSubKey("ns", "sp1", "key1");
+  auto key1 = prefs_.CreateSubKey({"ns", "sp1", "key1"});
   prefs_.AddObserver(key1, &mock_obserser);
 
   EXPECT_CALL(mock_obserser, OnPrefSet(key1));
@@ -424,7 +434,7 @@
   EXPECT_FALSE(prefs_.Exists(kKey));
   EXPECT_TRUE(prefs_.Delete(kKey));
 
-  auto key = prefs_.CreateSubKey("ns", "sp", "sk");
+  auto key = prefs_.CreateSubKey({"ns", "sp", "sk"});
   ASSERT_TRUE(prefs_.SetInt64(key, 0));
   EXPECT_TRUE(prefs_.Exists(key));
   EXPECT_TRUE(prefs_.Delete(kKey));