code: private void handleMirrorOutError(IOException ioe) throws IOException {
      LOG.info(datanode.getDatanodeInfo() + ": Exception writing block " +
               block + " namespaceId: " + namespaceId + " to mirror " + mirrorAddr + "\n" +
               StringUtils.stringifyException(ioe));
      if (Thread.interrupted()) { // shut down if the thread is interrupted
        throw ioe;
      } else { // encounter an error while writing to mirror
        // continue to run even if can not write to mirror
        // notify client of the error
        // and wait for the client to shut down the pipeline
        mirrorError = true;
      }
    }
docs: While writing to mirrorOut, failure to write to mirror should not
  affect this datanode.