code: func (s *Session) Fsync(async bool) error {
  	return s.Run(bson.D{{"fsync", 1}, {"async", async}}, nil)
  }
docs: // Fsync flushes in-memory writes to disk on the server the session
  // is established with. If async is true, the call returns immediately,
  // otherwise it returns after the flush has been made.