 @param delCandidates
   *          Candidates for deletion. For normal replication, this set is the
   *          same with availableReplicas. For striped blocks, this set is a
   *          subset of availableReplicas.
 @param delCandidates
   *          Candidates for deletion. For normal replication, this set is the
   *          same with availableReplicas. For striped blocks, this set is a
   *          subset of availableReplicas. | BlockPlacementPolicy.java