(First post in the Nginx forum, so please bear with me if it seems slightly long...)

I'm the maker of Engintron, a popular cPanel plugin which integrates Nginx as a reverse caching proxy in front of Apache. The project started in Dec 2014, it is open source & free and you can check out the code here, including Nginx configuration files: https://github.com/engintron/engintron

In Feb 2016 I rewrote the plugin from scratch using a more efficient universal proxy configuration in Nginx, for any domain hosted on a cPanel server. The benefit is that you install Engintron on your cPanel server and it just works, with zero maintenance and no vhost sync'ing between Nginx & Apache like similar plugins for cPanel.

I am defining 2 proxy cache zones/pools plus a proxy temp location in /etc/nginx/nginx.conf like this:

proxy_cache_path /tmp/engintron_dynamic levels=1:2 keys_zone=engintron_dynamic:20m inactive=10m max_size=500m;
proxy_cache_path /tmp/engintron_static levels=1:2 keys_zone=engintron_static:20m inactive=10m max_size=500m;
proxy_temp_path /tmp/engintron_temp;

Nginx does 1 min caching for static assets like CSS, JS, images etc. and 1 sec micro-caching for dynamic/HTML content. The latter is used so that it doesn't break sites with user-generated content such as forums, e-commerce sites etc.

In CentOS versions 5 & 6, the folders referenced above will start filling up when Nginx is installed via Engintron, as Nginx caching kicks in.
In CentOS 7 however - and this has been verified in multiple cPanel installations on my end, as well as other Engintron users via the project's Issues section on GitHub- the above folders are just empty.

However caching DOES work in CentOS 7, at least according to the headers sent by Nginx as well as by checking a static file if it gets cached.

For example if I modify this CSS file http://www.benetomaretti.com/templates/vp_merchant/css/template.css I won't see any changes until a minute has passed (1 min static cache). So the file does indeed get cached by Nginx, but the cache entry does not appear in the proxy cache/temp folders. These folders show up as empty with a simple du command.

And if you check the same file via curl, you get something like this:

$ curl -I http://www.benetomaretti.com/templates/vp_merchant/css/template.css
HTTP/1.1 200 OK
Server: nginx
Date: Wed, 06 Apr 2016 04:27:52 GMT
Content-Type: text/css
Content-Length: 45237
Connection: keep-alive
Vary: Accept-Encoding
Last-Modified: Wed, 06 Apr 2016 04:25:52 GMT
ETag: "45a86635-b0b5-52fc95b6d4120"
Expires: Fri, 06 May 2016 04:27:52 GMT
Cache-Control: max-age=2592000
X-Cache-Status: HIT
X-Server-Powered-By: Engintron
Pragma: public
Cache-Control: public
Vary: Accept-Encoding
Accept-Ranges: bytes

I'm really out of ideas why on earth there's this behaviour with CentOS 7 specifically.

Any pointers would be appreciated.

For the record, the (re-written) release of Engintron in Feb 2016 already has more than 1300 server deployments in 72 countries worldwide.

It's in my plans to also get anonymous system metrics at some point in the near future, so I know which CentOS version is used (along with other data such Apache version, MySQL version etc.). In the meantime I can't know what percentage uses CentOS 7, but judging from the fact that new cPanel deployments usually get setup on CentOS 7, I'm assuming the percentage may be high overall.

Thanks in advance for any help or guidance.

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Maker of Engintron - https://engintron.com
The easy way to integrate Nginx on cPanel as a reverse caching proxy that actually works for any type of website (even forums or e-shops).