/**
  * 
  * Get the size of the object-cache in bytes.
  * 
  * @param string $backend
  * @return int
  */
 private function getCacheSize($backend = 'file')
 {
     $size = 0;
     if ($backend === 'file') {
         if ($files = Cacheable::get_cache_files()) {
             foreach ($files as $file) {
                 $size += $file->Size;
             }
         }
         return $size;
     }
     return 'Not available.';
 }