func optimalPartSize(objectSize int64) (partSize int64, err error) {
	// object size is '-1' set it to 5TiB.
	if objectSize == -1 {
		objectSize = maxMultipartPutObjectSize
	}

	// object size is larger than supported maximum.
	if objectSize > maxMultipartPutObjectSize {
		err = errors.New("entity too large")
		return
	}

	configuredPartSize := minPartSize
	// Use floats for part size for all calculations to avoid