If enabled, this setting will use the database to store information about current sessions. Note that changing this setting now will log out all current users (including you). If you are using MySQL please make sure that 'max_allowed_packet' in my.cnf (or my.ini) is at least 4M. Other session drivers can be configured directly in config.php, see config-dist.php for more information. This option disappears if you specify session driver in config.php file.