That looks like it does a sync when the download is complete, which it should do. I can't say for sure without seeing the complete source code in context, but ISTM that "if(++_curr_download == _num_downloads)" means don't do anything until the current download is the final download. One write per download is a far different thing than one write per tile. I don't think any competent coder would do a file write for every map tile.