FYI, a common issue that people run into after setting up Magento for Redis Sessions is running into this error
Error: Warning: session_start(): Failed to read session data: user (path: /var/lib/php/sessions) in /var/www/html/vendor/magento/framework/Session/SessionManager.php on line 206
This happens when a user opens more than 6 sessions in quick succession. For example, clicking multiple links Frontend/Backend. The default configuration is max sessions per user is “6”. Which is very low and should be tweaked higher based on what your server can handle.
“12” seems to eliminate the issue without causing too much stress on the server. For more details on the problem and the fix, checkout this post: Error: Failed to read session data: user (path: /var/lib/php/sessions) from April.
It’s not something you’d come across normally until your site traffic starts to build up. So, just bare this in mind when the time comes.
Good luck.