How to configure nginx to serve static contents from RAM?

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http://serverfault.com – I want to set up nginx as my web server. I want to have image files cached in the memory (RAM) rather then disk. I am serving a small page and want few images always served from RAM. I dont wish to use varnish (or any other such tools) for this as I believe nginx has a capability to cache contents into RAM. I am not sure as how may I configure nginx for this? I did try few combinations but they didn't work. nginx uses disk all the time to get images. For example, when I tried apache benchmark to test with following command - ab -c 500 -n 1000 http://localhost/banner.jpg I get following err (HowTos)