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.
{
        if (fsync(fds[i]))
            throwFromErrno("Cannot fsync", ErrorCodes::CANNOT_FSYNC);
    }
    watch.stop();
    for (size_t i = 0; i < descriptors; ++i)
    {
        if (0 != close(fds[i]))
            throwFromErrno("Cannot close file", ErrorCodes::CANNOT_CLOSE_FILE);
    }
    std::cout << std::fixed << std::setprecision(2)
    << "Done " << count  << " ops" << " in " << watch.elapsedSeconds() << " sec."
    << ", " << count / watch.elapsedSeconds() << " ops/sec."
    << ", " << count * block_size / watch.elapsedSeconds() / 1000000 << " MB/sec."
    << std::endl;
    return 0;
}
int main(int argc, char ** argv)
{
    try
    {
        return mainImpl(argc, argv);
    }
    catch (const Poco::Exception & e)
    {
        std::cerr << e.what() << ", " << e.message() << std::endl;
        return 1;
    }
}