Well, for one thing, on some systems (e.g., OS X), /tmp is disk-backed. But also, by writing out to /tmp on memory-backed systems, the lack of blocking means y

Well, for one thing, on some systems (e.g., OS X), /tmp is disk-backed. But also, by writing out to /tmp on memory-backed systems, the lack of blocking means you grow memory use potentially indefinitely if the reader is slow or delayed for some reason. That will ultimately turn into swapping, which is just disk access again. There's also no great way to truncate the beginning of a regular file.