Preparing a server for Black Friday: PHP-FPM, MySQL and NGINX cache checklist

Black Friday 2026 falls on Friday, 27 November. Two weeks beforehand — by Friday, 13 November — it's time to open the terminal and check four points that, under load, make the difference between a slow site and a down site. None of the changes below require new hardware: they are configuration parameters that on many default installations remain those of a VPS with few daily visits.

1. PHP-FPM: pm.max_children

This is the maximum number of PHP processes the server can run simultaneously. If it's too low, excess requests queue up and TTFB increases; if it's too high, RAM fills before traffic subsides.

To calculate it, first measure how much RAM an active PHP-FPM process uses on average: the command ps -ylC php-fpm --sort:rss shows the memory used by each child process, with the RSS value expressed in kilobytes.

Then apply the formula: pm.max_children = total RAM available for PHP / average size of a PHP process. A practical example from the same source: with 2000 MB of RAM available and processes around 50 MB each, 2000/50 gives a pm.max_children of 40.

The file to modify is /etc/php/8.1/fpm/pool.d/www.conf on Ubuntu 22.04 (PHP 8.1 by default) and /etc/php/8.3/fpm/pool.d/www.conf on Ubuntu 24.04 (PHP 8.3 by default). After modification: systemctl reload php8.1-fpm (or php8.3-fpm).

2. MySQL/MariaDB: max_connections

Each PHP request querying the database opens a connection. Since 2010, with MySQL 5.5, the default value of max_connections is 151, the same value applies on MySQL 8.0.46 and MariaDB 10.11.14, the versions included in Ubuntu 24.04. During a spike of simultaneous checkouts, 151 runs out quickly.

Check with SHOW VARIABLES LIKE 'max_connections'; and raise it by modifying, on Ubuntu/Debian, the file /etc/mysql/mariadb.conf.d/50-server.cnf, adding under [mysqld] the line max_connections = 300 (or 500 if the server has sufficient RAM), followed by systemctl restart mariadb. Attention: each open connection consumes additional RAM, raise it alongside the pm.max_children check, not instead of it.

3. Static fallback page

If PHP-FPM saturates the workers, NGINX responds with 502. A static emergency page (even just HTML with the message "cart temporarily busy, try again in a few seconds") prevents the user seeing the naked error. In practice, add to the location block a try_files $uri @fallback; with a @fallback block that serves a static file from /var/www/maintenance.html when the backend doesn't respond within the timeout set with proxy_intercept_errors on; and error_page 502 503 504 /maintenance.html;.

4. CDN failover testing

Having a CDN configured is not enough: test it before the day. Temporarily disable the origin (or simulate a timeout) and verify with curl -I that the response arrives anyway, checking the cache header (e.g. X-Cache or your provider's equivalent) to understand whether the content is served from edge cache or fails along with the origin.

Summary

Check	Verification command	File to modify
pm.max_children	ps -ylC php-fpm --sort:rss	/etc/php/8.1(or 8.3)/fpm/pool.d/www.conf
max_connections	SHOW VARIABLES LIKE 'max_connections';	/etc/mysql/mariadb.conf.d/50-server.cnf
Static page	curl -I https://yoursite.it	location block in NGINX
CDN failover	curl -I with origin disabled	CDN provider panel

Each modification should be tested before 13 November, not on the day of the campaign launch.