// Now iterate through the blocks, reading them one at a time.
  for (size_t pos = start; pos < finish; pos += block_size_) {
    Key key = std::make_pair(filename, pos);
    // Look up the block, fetching and inserting it if necessary, and update the
    // LRU iterator for the key and block.
    std::shared_ptr<Block> block = Lookup(key);
    if (!block) {
      std::cerr << "No block for key " << key.first << "@" << key.second;