Nginx cache tuning / debug

[Simone Fumagalli]

Hello. I cache contents with NGINX proxy_cache in all my sites but I'm not sure I've the best setup.

Are there rules/hints to properly set the size and/or the TTL for the cache ? 
Would be nice to have a tool that monitor/track files written in the cache and those deleted (an the reason as well)

Is there such a tool or can I enable some logging feature to get these data ? 

Thanks

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Simone