  void start_split(spg_t parent, const set<spg_t> &children) {
    Mutex::Locker l(in_progress_split_lock);
    return _start_split(parent, children);
  }
  void start_split(spg_t parent, const set<spg_t> &children) {
    Mutex::Locker l(in_progress_split_lock);
    return _start_split(parent, children);
  }