code: public void scheduleFsync(long sequence, K key, Result<Boolean> result)
    {
      _requestSequence = Math.max(_requestSequence, sequence);
  
      if (sequence <= _tailSequence) {
        result.ok(Boolean.TRUE);
      }
      else {
        _storeFsync.addResult(key, result);
      }
    }
docs: Schedule an fsync for the given sequence, but do not request a flush.
  The fsync can occur before the flush, depending on other clients.
  
  @param sequence the sequence allocated from allocateFsyncSequence
  @param result notifies the caller when the fsync completes