В логе ошибок Nginx:

...socket() failed (24: Too many open files) while connecting to upstream..."

Список файловых дестрипторов, созданных процессом, можно увидеть в каталоге

/proc/$pid/fd

где

• $pid - ID процесса

Красивый вывод статистики по процессам Nginx:

for pid in `pidof nginx`; do echo "$(< /proc/$pid/cmdline)"; egrep 'files|Limit' /proc/$pid/limits; echo "Currently open files: $(ls -1 /proc/$pid/fd | wc -l)"; echo; done

nginx: worker process
Limit                     Soft Limit           Hard Limit           Units     
Max open files            131072               131072               files     
Currently open files: 25

nginx: worker process
Limit                     Soft Limit           Hard Limit           Units     
Max open files            131072               131072               files     
Currently open files: 29

nginx: worker process
Limit                     Soft Limit           Hard Limit           Units     
Max open files            131072               131072               files     
Currently open files: 29

nginx: worker process
Limit                     Soft Limit           Hard Limit           Units     
Max open files            131072               131072               files     
Currently open files: 55

nginx: master process /usr/sbin/nginx -c /etc/nginx/nginx.conf
Limit                     Soft Limit           Hard Limit           Units     
Max open files            1024                 4096                 files     
Currently open files: 23