public synchronized void restartWriterThreadsIfNeeded() {
    for (int i = 0; i < writerThreads.size(); i++) {
      WriterThread t = writerThreads.get(i);
      if (!t.isAlive()) {
        String threadName = t.getName();
        LOG.debug("Replacing dead thread: " + threadName);
        WriterThread newThread = new WriterThread(controller, entryBuffers, this, threadName);
        newThread.start();
        writerThreads.set(i, newThread);
      }
    }
  }
Start the threads that will pump data from the entryBuffers to the output files.
hbase-server/src/main/java/org/apache/hadoop/hbase/wal/OutputSink.java