Hello.
I would like to setup as many instance as necessary so I can stop only one database without stopping everything. So I have not configured [mysqld], and try to use only mysqld_multi.

Service File ( in /etc/systemd/system/mysqld@.service )

Code:
[Unit]
Description=MySQL Multi Server for instance %i
After=syslog.target
After=network.target

[Service]
User=mysql
Group=mysql
Type=forking
ExecStart=/usr/bin/mysqld_multi --defaults-file=/etc/mysql/cfg_%i.cnf --log=/var/log/mysql/mysql_multi.log start %i
ExecStop=/usr/bin/mysqld_multi --log=/var/log/mysql/mysql_multi.log stop %i
Restart=always
RestartSec=5
PrivateTmp=true

[Install]
WantedBy=multi-user.target
I have put the configuration file in /etc/mysql/cfg_1.cnf for my first instance.
I have put the configuration file in /etc/mysql/cfg_2.cnf for my second instance.

Here config file for instance 2 :

Code:
[mysqld2]
port = 63332
socket = /bdd/bd_2/sock/postfix.sock
pid-file = /bdd/bd_2/sock/postfix.pid
user = postfix
general_log = 1
general_log_file = /var/log/mysql/postfix/postfix.log
log-error = /var/log/mysql/postfix/postfix_error.log
log-warnings = 6
datadir = /bdd/bd_2/postfix
symbolic-links = 0
here is the general configuration file ( /etc/my.cnf )

Code:
[client]
port = 3306
socket = /bdd/bd_1/sock/mysql_cli.sock

[mysqld_multi]
mysqld = /usr/bin/mysqld_safe
mysqladmin = /usr/bin/mysqladmin
log = /var/log/mysql/mysqld_multi.log

!includedir /etc/my.cnf.d
I can start instance 1 but not instance 2

Here are screen logs :

Code:
linux:~ # systemctl start mysqld@1
linux:~ # systemctl status mysqld@1
mysqld@1.service - MySQL Multi Server for instance 1
Loaded: loaded (/etc/systemd/system/mysqld@.service; enabled)
Active: active (running) since Wed 2015-01-21 19:45:21 CET; 2s ago
Process: 5178 ExecStart=/usr/bin/mysqld_multi --defaults-file=/etc/mysql/cfg_%i.cnf --log=/var/log/mysql/mysql_multi.log --verbose start %i (code=exited, status=0/SUCCESS)
Main PID: 5184 (mysqld)
CGroup: /system.slice/system-mysqld.slice/mysqld@1.service
└─5184 /usr/sbin/mysqld --port=63331 --datadir=/bdd/bd_1/mysql --socket=/bdd/bd_1/sock/mysql.sock --pid-file=/bdd/bd_1/sock/mysql.pi...

Jan 21 19:45:21 linux systemd[1]: Started MySQL Multi Server for instance 1.
linux:~ # systemctl start mysqld@2
linux:~ # systemctl status mysqld@2
mysqld@2.service - MySQL Multi Server for instance 2
Loaded: loaded (/etc/systemd/system/mysqld@.service; enabled)
Active: inactive (dead) since Wed 2015-01-21 19:45:31 CET; 3s ago
Process: 5216 ExecStop=/usr/bin/mysqld_multi --log=/var/log/mysql/mysql_multi.log stop %i (code=exited, status=0/SUCCESS)
Process: 5212 ExecStart=/usr/bin/mysqld_multi --defaults-file=/etc/mysql/cfg_%i.cnf --log=/var/log/mysql/mysql_multi.log --verbose start %i (code=exited, status=0/SUCCESS)

Jan 21 19:45:31 linux systemd[1]: Started MySQL Multi Server for instance 2.
linux:~ #
Any help is welcome.

Thank you for helping
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