diff --git a/core/core.services.yml b/core/core.services.yml
index baf6fe4..918ec61 100644
--- a/core/core.services.yml
+++ b/core/core.services.yml
@@ -85,9 +85,15 @@ services:
   config.installer:
     class: Drupal\Core\Config\ConfigInstaller
     arguments: ['@config.factory', '@config.storage', '@config.typed', '@config.manager', '@event_dispatcher']
+  # Primary runtime config storage; injected as dependency into other services.
+  # Potentially cached.
   config.storage:
     alias: config.storage.active
+  # Actual storage used for runtime configuration; MUST NOT be cached.
+  # @see \Drupal\Core\DependencyInjection\UpdateServiceProvider
   config.storage.active:
+    alias: config.storage.database
+  config.storage.database:
     class: Drupal\Core\Config\DatabaseStorage
     arguments: ['@database', 'config']
   config.storage.file:
diff --git a/core/lib/Drupal/Core/Config/Storage/ChainWriteStorage.php b/core/lib/Drupal/Core/Config/Storage/ChainWriteStorage.php
new file mode 100644
index 0000000..ec58487
--- /dev/null
+++ b/core/lib/Drupal/Core/Config/Storage/ChainWriteStorage.php
@@ -0,0 +1,151 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Config\Storage\ChainWriteStorage.
+ */
+
+namespace Drupal\Core\Config\Storage;
+
+use Drupal\Core\Config\StorageInterface;
+
+/**
+ * Chained write configuration storage.
+ *
+ * This storage reads from and writes to a primary storage, and additionally
+ * writes to additional secondary storages.
+ */
+class ChainWriteStorage implements StorageInterface {
+
+  /**
+   * The primary configuration storage.
+   *
+   * @var \Drupal\Core\Config\StorageInterface
+   */
+  protected $storage;
+
+  /**
+   * The secondary configuration storage(s).
+   *
+   * @var \Drupal\Core\Config\StorageInterface
+   */
+  protected $secondary = array();
+
+  /**
+   * Constructs a new CachedStorage.
+   *
+   * @param \Drupal\Core\Config\StorageInterface $storage
+   *   The primary configuration storage.
+   */
+  public function __construct(StorageInterface $storage) {
+    $this->storage = $storage;
+  }
+
+  /**
+   * Adds a secondary storage to write to.
+   *
+   * @param \Drupal\Core\Config\StorageInterface $storage
+   *   The secondary configuration storage to add.
+   *
+   * @return $this
+   */
+  public function addStorage(StorageInterface $storage) {
+    $this->secondary[] = $storage;
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function exists($name) {
+    return $this->storage->exists($name);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function listAll($prefix = '') {
+    return $this->storage->listAll($prefix);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function read($name) {
+    return $this->storage->read($name);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function readMultiple(array $names) {
+    return $this->storage->readMultiple($names);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function write($name, array $data) {
+    if ($this->storage->write($name, $data)) {
+      foreach ($this->secondary as $storage) {
+        $storage->write($name, $data);
+      }
+      return TRUE;
+    }
+    return FALSE;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function delete($name) {
+    if ($this->storage->delete($name)) {
+      foreach ($this->secondary as $storage) {
+        $storage->delete($name);
+      }
+      return TRUE;
+    }
+    return FALSE;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function deleteAll($prefix = '') {
+    if ($this->storage->deleteAll($prefix)) {
+      foreach ($this->secondary as $storage) {
+        $storage->deleteAll($prefix);
+      }
+      return TRUE;
+    }
+    return FALSE;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function rename($name, $new_name) {
+    if ($this->storage->rename($name, $new_name)) {
+      foreach ($this->secondary as $storage) {
+        $storage->rename($name, $new_name);
+      }
+      return TRUE;
+    }
+    return FALSE;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function encode($data) {
+    return $this->storage->encode($data);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function decode($raw) {
+    return $this->storage->decode($raw);
+  }
+
+}
diff --git a/core/lib/Drupal/Core/DrupalKernel.php b/core/lib/Drupal/Core/DrupalKernel.php
index 8812cab..7582e28 100644
--- a/core/lib/Drupal/Core/DrupalKernel.php
+++ b/core/lib/Drupal/Core/DrupalKernel.php
@@ -246,6 +246,9 @@ public function discoverServiceProviders() {
     if (!empty($GLOBALS['conf']['container_yamls'])) {
       $this->serviceYamls = array_merge($this->serviceYamls, $GLOBALS['conf']['container_yamls']);
     }
+    if (file_exists($site_services_yml = conf_path() . '/services.yml')) {
+      $this->serviceYamls[] = $site_services_yml;
+    }
     return $serviceProviders;
   }
 
diff --git a/sites/default/default.settings.php b/sites/default/default.settings.php
index b2ce930..8159676 100644
--- a/sites/default/default.settings.php
+++ b/sites/default/default.settings.php
@@ -244,6 +244,35 @@
  */
 
 /**
+ * Active/runtime configuration storage settings.
+ *
+ * By default, the active configuration is stored in the {config} database
+ * table. To use or install with a different storage, override the
+ * 'bootstrap_config_storage' setting.
+ *
+ * $settings['bootstrap_config_storage'] specifies a callable that instantiates
+ * a storage class implementing \Drupal\Core\Config\StorageInterface. This
+ * storage must be able to read and return the current 'core.extension'
+ * configuration.
+ *
+ * In addition, you need to override the 'config.storage.active' service
+ * definition in services.yml of a profile, module, or the site.
+ *
+ * Alternatively, you can use the default storage, and only write to a secondary
+ * storage, by putting the following into the services.yml file in the site
+ * directory:
+ * @code
+ * services:
+ *   config.storage:
+ *     class: Drupal\Core\Config\Storage\ChainWriteStorage
+ *     arguments: ['@config.storage.active']
+ *     calls:
+ *       - [addStorage, ['@config.storage.file']]
+ * @endcode
+ */
+# $settings['bootstrap_config_storage'] = 'Drupal\Core\Config\BootstrapConfigStorageFactory::getFileStorage';
+
+/**
  * Salt for one-time login links, cancel links, form tokens, etc.
  *
  * This variable will be set to a random value by the installer. All one-time
@@ -601,19 +630,6 @@
 # $cookie_domain = '.example.com';
 
 /**
- * Active configuration settings.
- *
- * By default, the active configuration is stored in the database in the
- * {config} table. To install Drupal with a different active configuration
- * storage, you need to override the setting here, in addition to overriding
- * the config.storage.active service definition in a module or profile.
- *
- * The 'bootstrap_config_storage' setting needs to be a callable that returns
- * core.services.yml.
- */
- # $settings['bootstrap_config_storage'] = array('Drupal\Core\Config\BootstrapConfigStorageFactory', 'getFileStorage');
-
-/**
  * Configuration overrides.
  *
  * To globally override specific configuration values for this site,