nginx cache growing well above max_size threshold

[Maxim Dounin]

Hello!

On Thu, Sep 14, 2017 at 04:34:09PM +0000, Lucas Rolff wrote:

> I have a minor question, so I have an nginx box using 
> proxy_cache, it has a key zone of 40 gigabyte (so it can cache 
> 320 million files), a max_size of 1500 gigabyte for the cache 
> and the inactive set to 30 days.
> 
> However we experience that nginx goes well above the defined 
> limit - in our case the max size is 1500 gigabyte, but the cache 
> directory takes goes well above 1700 gigabyte.
> 
> There's a total of 42.000.000 files currently on the system, 
> meaning the average filesize is about 43 kilobyte.
> 
> Normally I know that nginx can go slightly above the limit, 
> until the cache manager purges the files, but it stays at about 
> 1700 gigabyte constantly unless we manually clear out the size.
> 
> I see there's a change in 1.13.1 that ignores long locked cache 
> entries, is it possible that this bugfix actually fixes above 
> issue?
> 
> Upgrading is rather time consuming and we have to ensure nginx 
> versions across the platform, so I wonder if anyone has some 
> pointers if the above bugfix would maybe solve our issue. 
> (currently the custom nginx version is based on nginx 1.10.3).

https://trac.nginx.org/nginx/ticket/1163

TL;DR:

This behaviour indicate there is a problem somewhere, likely 
socket leaks or process crashes.  Reports suggests it might be 
related to HTTP/2.  The change in 1.13.1 don't fix the root cause, 
but will allow nginx to keep cache under max_size regardless of 
the problem.

-- 
Maxim Dounin
http://nginx.org/