public Disk getRandomDisk() {
      DataNode dataNode = getRandomDataNode();
      if (dataNode == null || dataNode.getDisks().size() == 0) {
        return null;
      }
      return dataNode.getDisks().get(new Random().nextInt(dataNode.getDisks().size()));
    }
Get given number of datanodes from each datacenter.
@param dataNodeCountPerDc number of datanodes to get from each datacenter.
@return a list of datanodes.
ambry-clustermap/src/test/java/com/github/ambry/clustermap/TestUtils.java