void startWriterThreads() throws IOException {
    for (int i = 0; i < numThreads; i++) {
      WriterThread t = new WriterThread(controller, entryBuffers, this, i);
      t.start();
      writerThreads.add(t);
    }
  }
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