/**
   * choose <i>numOfReplicas</i> data nodes for <i>writer</i> 
   * to re-replicate a block with size <i>blocksize</i>
/**
   * choose <i>numOfReplicas</i> data nodes for <i>writer</i> 
   * to re-replicate a block with size <i>blocksize</i> | BlockPlacementPolicy.java