code: func (si *SegmentCommitInfo) SizeInBytes() (sum int64, err error) {
  	if si.sizeInBytes == -1 {
  		sum = 0
  		for _, fileName := range si.Files() {
  			d, err := si.Info.Dir.FileLength(fileName)
  			if err != nil {
  				return 0, err
  			}
  			sum += d
  		}
  		si.sizeInBytes = sum
  	}
  	return si.sizeInBytes, nil
  }
docs: /*
  Returns total size in bytes of all files for this segment.
  
  NOTE: This value is not correct for 3.0 segments that have shared
  docstores. To get correct value, upgrade.
  */