caching issues with nginx as reverse-proxy

[dm at whocaresabout.de]

Hi community,

currently I am serving files with a size about 1,5G (static without 
dynamic content) using a hand full of nodes and nginx in reverse proxy 
setup.
Caching works, ..but not as expected. During the requests nginx creates 
a lot of temp caching files that grow up to the size of the origin file 
delivered from backend server. It's kinda weird that even if the 
inactive caching time out is not reached, the file is randomly 
downloaded again from backend.

I guess that I just simply got blind by the configure that affects that.

## /etc/nginx/nginx.conf ##
proxy_cache_path /var/tmp/nginx-cache/rproxy levels=1:2 
keys_zone=rproxy:260m max_size=5g inactive=260m use_temp_path=off;
proxy_cache_key "$scheme$request_method$host$request_uri$is_args$args";
proxy_cache_valid 200 302 45m;
proxy_cache_valid 404 1m;

## /etc/nginx/sites-available/domain.tld ##
server {
     listen 80;# default_server;
     listen [::]:80;# default_server;

     server_name domain.tld;

     location / {
     return 301 https://$host$request_uri;
     }

}

     server {

     listen 443 ssl http2; #default_server;
     listen [::]:443 ssl http2; #default_server;

     ssl		on;
     ssl_certificate		/etc/letsencrypt/live/domain.tld/fullchain.pem;
     ssl_certificate_key		/etc/letsencrypt/live/domain.tld/privkey.pem;

     server_name domain.tld;

     location / {
#	proxy_buffering on;
	proxy_cache_revalidate on;
	proxy_cache_lock on;
	proxy_cache rproxy;
	proxy_cache_use_stale error timeout http_500 http_502 http_503 
http_504;
	proxy_pass https://rproxy;
     }

}
####

Nginx 1.11.1 is used. And the frontends got only 1gb ram and 10 gb of 
disk space.

I am looking forward for any advice that could bring me some steps 
forward to the right configuration :-)

Thanks in advance!

Best regards,
Daniel