nginx configured to cache but seems not to be caching

I've configured a basic caching on my nginx server, but when I go to the cache storage folder, no cache files are visible. Here's my configuration:

In nginx.conf:

    proxy_cache_path  /usr/share/nginx/www/cache levels=1:2 keys_zone=my-cache:8m max_size=1000m inactive=600m;
    proxy_temp_path /usr/share/nginx/www/cache/tmp;

And in sites-available/default:

upstream app_mydomain {
        server 127.0.0.1:3000;
}

server {
        listen   80;

        server_name mydomain.com;

        location / {
                proxy_set_header X-Real-IP $remote_addr;
                proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
                proxy_set_header Host $http_host;
                proxy_set_header X-NninX-Proxy true;

                proxy_pass http://app_mydomain/;
                proxy_redirect off;

                proxy_cache my-cache;
        }
}

The permissions for the cache directory are 700, owner www-data. What do I need to add to get it caching?