public static String store2Hdfs(Key srcKey) {
    assert srcKey._kb[0] != Key.ARRAYLET_CHUNK;
    assert PersistHdfs.getPathForKey(srcKey) != null; // Validate key name
    Value v = DKV.get(srcKey);
    if (v == null) return "Key " + srcKey + " not found";
    if (v._isArray == 0) { // Simple chunk?
      v.setHdfs(); // Set to HDFS and be done
      return null; // Success
    }

    // For ValueArrays, make the .hex header
    ValueArray ary = ValueArray.value(v);
    String err = PersistHdfs.freeze(srcKey, ary);
    if (err != null) return err;

    // The task managing which chunks to write next,
    // store in a known key
    TaskStore2HDFS ts = new TaskStore2HDFS(srcKey);
    Key selfKey = ts.selfKey();
    UKV.put(selfKey, ts);

    // Then start writing chunks in-order with the zero chunk
    H2ONode chk0_home = ValueArray.getChunkKey(0, srcKey).home_node();
    RPC.call(ts.chunkHome(), ts);

    // Watch the progress key until it gets removed or an error appears
    long idx = 0;
    while (UKV.get(selfKey, ts) != null) {
      if (ts._indexFrom != idx) {
        System.out.print(" " + idx + "/" + ary.chunks());
        idx = ts._indexFrom;
      }
      if (ts._err != null) { // Found an error?
        UKV.remove(selfKey); // Cleanup & report
        return ts._err;
      }
      try {
        Thread.sleep(100);
      } catch (InterruptedException e) {
      }
    }
    System.out.println(" " + ary.chunks() + "/" + ary.chunks());

    // PersistHdfs.refreshHDFSKeys();
    return null;
  }
  @Override
  public void compute() {
    String path = null; // getPathFromValue(val);
    ValueArray ary = ValueArray.value(_arykey);
    Key self = selfKey();

    while (_indexFrom < ary.chunks()) {
      Key ckey = ary.getChunkKey(_indexFrom++);
      if (!ckey.home()) { // Next chunk not At Home?
        RPC.call(chunkHome(), this); // Hand the baton off to the next node/chunk
        return;
      }
      Value val = DKV.get(ckey); // It IS home, so get the data
      _err = PersistHdfs.appendChunk(_arykey, val);
      if (_err != null) return;
      UKV.put(self, this); // Update the progress/self key
    }
    // We did the last chunk.  Removing the selfKey is the signal to the web
    // thread that All Done.
    UKV.remove(self);
  }