@Override
  public void waitForFinish() {
    super.waitForFinish();
    System.out.println("Failed to update keys: " + failedKeySet.size());
    for (Long key : failedKeySet) {
      System.out.println("Failed to update key: " + key);
    }
  }
Use batch vs. separate updates for every column in a row
hbase-diagnostics/src/main/java/org/apache/hadoop/hbase/util/MultiThreadedUpdater.java