private function checkThatHostnameIsValidOnUpdate($id, $hostname)
 {
     if (!$hostname) {
         return true;
     }
     if ($this->dao->getNumberOfMirrorByHostnameExcludingGivenId($hostname, $id) > 0) {
         throw new Git_Mirror_HostnameAlreadyUsedException();
     }
     if ($this->doesHostnameIsForbidden($hostname)) {
         throw new Git_Mirror_HostnameIsReservedException();
     }
     return true;
 }