protected function getSize()
    {
        if (! $this->size) {
            $size = $this->getParam(self::MAX_VALUE_SIZE);
            if ($size !== false) {
                if (!is_int($size)) {
                    throw new common_Exception('Persistence max value size has to be an integer');
                }
                $this->size = $size - strlen($this->getMapIdentifier());
            }
        }
        return $this->size;
    }