[myself248]

Out of curiosity, do you know off-hand how torrent clients do it on filesystems that don't support sparse files? There must be either a preallocate-the-whole-thing step, or a gather-the-pieces-together-and-write-out-the-large-file step. The latter would seem to briefly double the disk space needed at the end of the download, so I suspect they do the former.

[ajross]

Chunk it up and resassemble, one assumes. Things aren't nearly as clear in the modern world of gigabit pipes into suburban households[1], but when these things were written the filesystem was 100x faster than the link to those peer connections from which the data was fetched. A final copy was only a small overhead.

[1] Which is why all the stuff we used to torrent is in the cloud now.

[cesarb]

AFAIK, they preallocate; and even on filesystems which support sparse files, most bittorrent clients have an option to always preallocate (to both reserve space and reduce fragmentation).