code: private Block[] allocateParityBlocks(int numParityBlocks) throws IOException {
      Block[] blocks = new Block[numParityBlocks];
      for (int i = 0; i < numParityBlocks; i++) {
        Block b = new Block(generateBlockId(), 0, 0);
        while (isValidBlock(b)) {
          b.setBlockId(generateBlockId());
        }
        b.setGenerationStamp(getGenerationStamp());
        blocks[i] = b;
      }
      return blocks;
    }
docs: Allocate a number of parity blocks
  Require a write lock
  @param numParityBlocks