private function getMetaDataStorage()
    {
        $metaStorage = $this->options->metaStorage;
        if ( $metaStorage === null )
        {
            $metaStorage = reset( $this->storageStack )->storage;
            if ( !( $metaStorage instanceof ezcCacheStackMetaDataStorage ) )
            {
                throw new ezcBaseValueException(
                    'metaStorage',
                    $metaStorage,
                    'ezcCacheStackMetaDataStorage',
                    'top of storage stack'
                );
            }
        }
        return $metaStorage;
    }