nginx fails to start/reload on a Plesk server: Too many open files

Symptoms

• The nginx service fails to start/reload with the following error in the /var/log/nginx/error.log file:

  nginx: [emerg] open() "/var/www/vhosts/system/example.com/logs/proxy_access_ssl_log" failed (24: Too many open files)

• Some or all websites are not accessible

• Warning is shown in the Tools & Settings > Diagnose & repair:

  Checking for nginx ULIMIT value N domains have been found on server. Problem with nginx default limit for open files is possible

• NodeJS and Ruby web applications are showing 502 Bad Gateway error when performing some Plesk tasks such as creating domains, subscriptions, changes on Apache or Nginx, etc:

  Server Error
  502 Bad Gateway
  Web server received an invalid response while acting as a gateway or proxy server.

• After installing a new certificate for the domain, the old one is still shown in a web browser, however, the correct certificate is selected at the Domains > example.com > Hosting Settings page.

Cause

Nginx's virtual host opens 4 log files for each virtual host with physical hosting:

• proxy_access_log
• proxy_access_ssl_log
• webmail_access_log
• webmail_access_ssl_log

As a result of this fact, the maximum number of open files is exceeded by the nginx server due to a big number of domains (usually, more than 100).

Resolution

Warning: Execution of the websrv_ulimits utility without --no-restart option initiates rebuilding of web configuration files for all domains and can cause significant downtime in case of a big number of hosted websites.

Execute the command during the maintenance time frame.

It is good practice to set open files limit value more than number of domains in Plesk * 16 at least. In example below provided actions how to set limit to 4096:

1. Connect to the server via SSH

2. Set open files limit to 4096 for both Apache and Nginx:

   # /usr/local/psa/admin/sbin/websrv_ulimits --set 4096 --no-restart

3. Restart Apache and nginx services at Plesk > Tools & Settings > Services Management to apply changes.

4. Check that 'Max open files limit' was applied properly, after web services restarting:

   # grep 'Max open files' /proc/$(cat /var/run/nginx.pid)/limits
   Max open files 4096 4096 file

5. If the solution above did not help, set the limit using the ulimit utility:

   # ulimit -n 4096