Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/modules/file/src/FileStorageInterface.php

@@ -0,0 +1,31 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\file\FileStorageInterface.
+ */
+
+namespace Drupal\file;
+
+use Drupal\Core\Entity\EntityStorageInterface;
+
+/**
+ * Defines an interface for file entity storage classes.
+ */
+interface FileStorageInterface extends EntityStorageInterface {
+
+  /**
+   * Determines total disk space used by a single user or the whole filesystem.
+   *
+   * @param int $uid
+   *   Optional. A user id, specifying NULL returns the total space used by all
+   *   non-temporary files.
+   * @param int $status
+   *   (Optional) The file status to consider. The default is to only
+   *   consider files in status FILE_STATUS_PERMANENT.
+   *
+   * @return int
+   *   An integer containing the number of bytes used.
+   */
+  public function spaceUsed($uid = NULL, $status = FILE_STATUS_PERMANENT);
+}