mysqld服务启动失败， Failed to restart mysqld.service: Unit not found.

-bash-4.2# service mysqld restart
Redirecting to /bin/systemctl restart mysqld.service
Failed to restart mysqld.service: Unit not found.

并不存在 mysqld 的服务，

-bash-4.2#
-bash-4.2# chkconfig -list
-list: unknown option
-bash-4.2# chkconfig --list

Note: This output shows SysV services only and does not include native
      systemd services. SysV configuration data might be overridden by native
      systemd configuration.

      If you want to list systemd services use 'systemctl list-unit-files'.
      To see services enabled on particular target use
      'systemctl list-dependencies [target]'.

aegis           :off   :off   :on    :on    :on    :on    :off
agentwatch      :off   :off   :on    :on    :on    :on    :off
iprdump         :off   :off   :on    :on    :on    :on    :off
iprinit         :off   :off   :on    :on    :on    :on    :off
iprupdate       :off   :off   :on    :on    :on    :on    :off
jexec           :off   :on    :on    :on    :on    :on    :off
mysql.server    :off   :off   :on    :on    :on    :on    :off
netconsole      :off   :off   :off   :off   :off   :off   :off
network         :off   :off   :on    :on    :on    :on    :off

可见，服务名不是mysqld 而是 mysql.server

于是：

-bash-4.2# service mysql.server restart
Shutting down MySQL....[ OK ]
Starting MySQL.[ OK ]