In these sorts of patterns I would also checkout Stream.into(File.stream!(path))

I’ve used File.open and IO.write / IO.binwrite with a Stream like you, dozens of times, and never benchmarked it against having File.write at each step. I simply assumed the latter is a terrible idea and can never be faster. Main reason might not even be speed per se; it’s just that opening and closing a file can be risky under load and in more constrained machines (which, let’s be real, are most cloud offerings out there, even the mainstream ones; they regularly put your apps and databases on I/O-throttled machines) – it might actually fail with “out of memory” or “not enough file handles” etc., I’ve seen those even on machines with 64GB of RAM and 2TB SSD storage.

TL;DR do as little syscalls as at all possible. We can’t optimize adequately for that when we work with a higher-level language with Elixir but at least we can use the little tools we have, like “don’t open and close file 500 times in a loop”.

…And all of that changes if each step to write to the file is seconds or minutes apart, of course (i.e. you stream data from a slower remote node).

I’m not sure I understand the question. If you need to do multiple writes to a file in a loop then yes, File.open is made for it. If you have the complete content of the file to write already, then use File.write only once.