code: public void flush(boolean durable) throws IOException {
      numSync++;
      long start = System.nanoTime();
      flushAndSync(durable);
      long time = DFSUtil.getElapsedTimeMicroSeconds(start);
      totalTimeSync += time;
      if (sync != null) {
        sync.inc(time);
      }
    }
docs: Flush data to persistent store.
  Collect sync metrics.