Solution how to fix following error generated in /var/log/syslog by mysqld service on Ubuntu 16.04 with MariaDB 10.0.x:

mysqld: 181210 [ERROR] Error in accept: Too many open files

If you see message above you probably reached default limits of mysqld service. To set open files parameter limit as unlimited, follow steps below:

• Login as root:

sudo -i

• Modify /etc/security/limits.conf:

nano /etc/security/limits.conf

• And set mysql soft and hard limit for nofiles:

...

mysql soft nofile 65535
mysql hard nofile 65535

# End of file

• Save modified limits.conf and Reboot system:

sudo reboot

• Create mysql.service.d folder to customize startup parameters and create limits.conf file:

mkdir -p /etc/systemd/system/mysql.service.d/
nano /etc/systemd/system/mysql.service.d/limits.conf

• Into limits.conf insert following:

[Service]
LimitNOFILE=infinity

• Save file and restart mysqld service:

systemctl daemon-reload
/etc/init.d/mysql restart

• Check new limits of mysqld process:

cat /proc/$(cat /var/run/mysqld/mysqld.pid)/limits

• Desired state after change:

Limit                     Soft Limit           Hard Limit           Units
...
Max open files            1048576              1048576              files
...

• By default mariadb has following:

Limit                     Soft Limit           Hard Limit           Units
...
Max open files            1024                 4096                 files
...