File size is updated correctly according to 

> fdatasync() is similar to fsync(), but does not flush modified metadata unless that metadata is needed in order to allow a subsequent data retrieval to be correctly handled.  For example, changes to  st_atime  or  st_mtime  (respec‐
       tively,  time  of  last  access  and  time  of last modification; see inode(7)) do not require flushing because they are not necessary for a subsequent data read to be handled correctly.  On the other hand, a change to the file size
       (st_size, as made by say ftruncate(2)), would require a metadata flush.

And we don't care about modification times.
Should work alright.
blocks_to_request.emplace_back(cell.index.block_index);
                    break;
                }
                case Cell::default_value:
                {
                    result.key_index_to_state[key_index] = {key_state, fetched_columns_index};
                    result.key_index_to_state[key_index].setDefault();
                    ++fetched_columns_index;
                    ++result.default_keys_size;
                    insertDefaultValuesIntoColumns(result.fetched_columns, fetch_request, key_index);
                    break;
                }
            }
        }
        
        std::sort(blocks_to_request.begin(), blocks_to_request.end());
        file_buffer.fetchBlocks(configuration.read_buffer_blocks_size, blocks_to_request, [&](size_t block_index, char * block_data)
        {
            auto & keys_in_block = block_to_keys_map[block_index];
            for (auto & key_in_block : keys_in_block)
            {
                char * key_data = block_data + key_in_block.offset_in_block;
                deserializeAndInsertIntoColumns(result.fetched_columns, fetch_request, key_data);
                result.key_index_to_state[key_in_block.key_index].setFetchedColumnIndex(fetched_columns_index);
                ++fetched_columns_index;
            }
        });
        return result;
    }
    void insertColumnsForKeysImpl(const PaddedPODArray<KeyType> & keys, Columns columns)