code: private int getQuotaForThisPriority(int totalQuota, int blocksForThisPriority,
        int blocksForLowerPriorities) {
      // reserve at most 20% for lower priority blocks
      int quotaForLowerPriorities = 
                       Math.min(totalQuota/5, blocksForLowerPriorities);
      return Math.min( blocksForThisPriority,
                       totalQuota-quotaForLowerPriorities);
    }
docs: Decide the number of blocks to replicate on for this priority.
  The heuristic is that allocate at most 20% the quota for
  lower priority blocks
  
  @param totalQuota max number of blocks can be replicated
  @param blocksForThisPriority
  number of needed replication blocks for this priority
  @param blocksForLowerPriorities
  number of lower priority needed replication blocks
  @return the number blocks