How can I tune NGINX static file caching in RAM?

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http://serverfault.com – From what I've read both NGINX and OS will cache static files being served in memory. Is there a way to set how much RAM NGINX is allowed to use? My web server will be exclusively serving a large amount of ephemeral static files from disk and I want these to be served from RAM when possible. I also saw a post that claimed reading from disk will block an entire nginx worker. Is this true? (HowTos)