Disable filesystem read cache
I think you might find what you're looking for in this superuser answer: https://superuser.com/questions/242928/disable-linux-read-and-write-file-cache-on-partition
The code linked from that answer is no longer available at code.google.com though, but it has been cloned to github: https://github.com/svn2github/pagecache-management
The instructions in that answer should still work though. Hope this helps!