public void delaySendingAckToUpstream(final String upstreamAddr)
      throws IOException {
  }
@Test(timeout = 60000)
  public void testDelaySendingAckToUpstream() throws Exception {
    final MetricsDataNodeFaultInjector mdnFaultInjector =
        new MetricsDataNodeFaultInjector() {
          @Override
          public void delaySendingAckToUpstream(final String upstreamAddr)
              throws IOException {
            delayOnce();
          }

          @Override
          public void logDelaySendingAckToUpstream(final String upstreamAddr,
              final long delayMs) throws IOException {
            logDelay(delayMs);
          }
        };
    verifyFaultInjectionDelayPipeline(mdnFaultInjector);
  }