"too many open files"
Parce que ulimit et /etc/security/limits.conf ne fonctionne pas avec systemd :

[Service]
Type=simple
User=farmer
....
LimitNOFILE=500000:500000
LimitNPROC=500000:500000

Sans oublier :

systemctl daemon-reload

Sur mon système qui n'a pas apparmor, mysqld démarré en ligne de commande, mais pas via systemd.
Systemd a des fonctionnalités pour protéger certains filesystem :

Change variable "ProtectHome" by "false" on "/etc/systemd/system/mysqld.service"
Reload systemd by "sudo systemctl daemon-reload"

Et pour ne pas perdre ses modifs, suivre plutôt ce lien : https://dba.stackexchange.com/questions/185302/mysql-mariadb-cannot-change-default-datadir-on-debian-9-1-server/191174

"According to Debian policy rules, you should set this option value in a custom file like /etc/systemd/system/mariadb.service.d/YOUR_CUSTOM_FILE.conf containing:"

[Service]

# Prevent accessing /home, /root and /run/user
ProtectHome=false

Savoir si le système utilise initd ou systemd

pidof /sbin/init && echo "sysvinit" || echo "other"
pidof systemd && echo "systemd" || echo "other"

Créer un fichier test.service, à mettre dans /usr/lib/systemd/system/

[Unit]
Description=Callcenter Service

[Service]
Type=oneshot
RemainAfterExit=no
User=<user>
ExecStart=<command>
ExecStop=<command>

[Install]
WantedBy=multi-user.target

Démarrer le service test

sudo systemctl start test.service

Activer le service test au démarrage

sudo systemctl enable startup.service

Lister les services démarrés

systemctl list-unit-files --state=enabled

Useful SystemD commands (hints for systemctl or systemctl vs chkconfig and service)
List all running services

systemctl

Start/stop or enable/disable services

systemctl start foo.service

Deactivates a service immediately:

systemctl stop foo.service

Restarts a service:

systemctl restart foo.service

Shows status of a service including whether it is running or not:

systemctl status foo.service

Enables a service to be started on bootup:

systemctl enable foo.service

Disables a service to not start during bootup:

systemctl disable foo.service

Check whether a service is already enabled or not:

systemctl is-enabled foo.service; echo $?

Des explications sur systemd. (via http://dooby.fr/links/?zNjMPw)

dans /var/log/journal/journal.conf :

SystemMaxUse=100M

Stockage (par défaut) dans : /var/log/journal

sudo journalctl --disk-usage
sudo journalctl --vacuum-size=100M

see also : http://unix.stackexchange.com/questions/139513/how-to-clear-journalctl

Explorer le journal :

Show all messages from date (and optional time):

# journalctl --since="2012-10-30 18:17:16"

Show all messages since 20 minutes ago:

# journalctl --since "20 min ago"

Follow new messages:

# journalctl -f

Show all messages by a specific executable:

# journalctl /usr/lib/systemd/systemd