The default open files limit is 1024. This is fine for a few websites, but if you are hosting many websites this should be increased.

1. Login as root via SSH

2. Type : pico /etc/systemd/system/mariadb.service.d/migrated-from-my.cnf-settings.conf

3. Change the lines to:
LimitNOFILE=100000
LimitMEMLOCK=100000

4. Execute :
systemctl daemon-reload
systemctl restart mysql

5. Type: pico /etc/security/limits.conf

Add/Modify the lines so they read:

* soft nofile 1024000
* hard nofile 1024000
* soft nproc 10240
* hard nproc 10240

Save: CTRL O

Quit: CTRL X

6. Type: pico /etc/security/limits.d/90-nproc.conf

Add/Modify the lines so they read:

* soft nofile 1024000
* hard nofile 1024000
* soft nproc 10240
* hard nproc 10240
root soft nproc unlimited

Save: CTRL O

Quit: CTRL X

7. Type: ulimit -Hn 1024000

8. Type: pico /etc/my.cnf

Insert the following:

open_files_limit = 1024000

If you have sections [mysqld] and [mysqld_safe] put the line under both headings.

Save: CTRL O

Quit: CTRL X

9. Type: service mysql restart