These twin arrangements look really interesting. Puch experimented with pairing up pistons inside a shared combustion chamber with a Y-shaped conrod: https://engineerine.com/the-forgotten-twingle-engine/
4,706 karma · joined May 11, 2015
As far as fragmentation goes, I think both Firefox and Chrome derivatives figure out the final file size in advance, pre-allocate a temporary file that long and put data in it as it arrives. Completing the download is a rename operation or a no-op.
Quick edit: hmm, I'm already ~1 hour late, the relevant thread starts at https://news.ycombinator.com/item?id=49327457