code: func GetBlockDeviceSize(file *os.File) (uint64, error) {
  	size, err := ioctlBlkGetSize64(file.Fd())
  	if err != nil {
  		logrus.Errorf("devicemapper: Error getblockdevicesize: %s", err)
  		return 0, ErrGetBlockSize
  	}
  	return uint64(size), nil
  }
docs: // GetBlockDeviceSize returns the size of a block device identified by the specified file.