code: func (d *Datastore) DiskUsage() (uint64, error) {
  	if d.path == "" { // in-mem
  		return 0, nil
  	}
  
  	var du uint64
  
  	err := filepath.Walk(d.path, func(path string, info os.FileInfo, err error) error {
  		if err != nil {
  			return err
  		}
  		du += uint64(info.Size())
  		return nil
  	})
  
  	if err != nil {
  		return 0, err
  	}
  
  	return du, nil
  }
docs: // DiskUsage returns the current disk size used by this levelDB.
  // For in-mem datastores, it will return 0.