Xenforo [alert] 29123#0: *119322 zero size buf in output t:0 r:0 f:1

never seen that error before although live update addon was hanging up my php-fpm unix socket connections with updates frequency being too fast <20 seconds

 

i raised the frequency of checks from original 10 seconds to 20 seconds but switching from php-fpm pool unix sockets back to TCP fixed my hanging issue but never experienced your error

from http://mailman.nginx.org/pipermail/nginx-devel/2014-January/004740.html suggests that error is harmless and occurs when request has an empty response (
Content-Length: 0)

You using default Centmin Mod TCP php-fpm single instance or started playing with load balanced upstreams ?

 

if you have a PHP intensive site/load then yeah but only as far as you having enough cpu threads to service the php-fpm workers. If you don't have enough cpu threads, having more php-fpm workers than cpu threads tends to do the opposite and slow things down