code: func (f *Field) loadAvailableShards() error {
  	bm := roaring.NewBitmap()
  	// Read data from meta file.
  	path := filepath.Join(f.path, ".available.shards")
  	buf, err := ioutil.ReadFile(path)
  	if os.IsNotExist(err) {
  		return nil
  	} else if err != nil {
  		return errors.Wrap(err, "reading available shards")
  	} else {
  		if err := bm.UnmarshalBinary(buf); err != nil {
  			return errors.Wrap(err, "unmarshaling")
  		}
  	}
  	// Merge bitmap from file into field.
  	f.mergeRemoteAvailableShards(bm)
  
  	return nil
  }
docs: // loadAvailableShards reads remoteAvailableShards data for the field, if any.