Nginx proxy cache/temp folders empty when located inside /tmp on CentOS 7

[fevangelou]

(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.

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,265943,265943#msg-265943