POSIX allows us to pass hints to the kernel about proceeding activity on a file. Specifically, it allows us to recommend aggressive readahead or none at all. The Linux kernel developers (notably Andrew Morton) have suggested that Postgres ought to be using posix_fadvise if it expects to get good readahead behavior from the kernel, so it seems at least worth considering (and benchmarking).

Here's a brief patch which adds these hints to the system. It probably needs to be brought up to HEAD.

http://www.alcove.com.au/~swm/fadvise.diff