环境：
OS：Ubuntu 14.04.1 LTS
Kernal：3.13.0-93-generic
过程：
apt-get install mysql-server
安装完后，修改了/etc/mysql/my.cf中的datadir指向一个挂载的硬盘上的目录，然后重启服务。
/etc/init.d/mysql restart
提示启动failed，查看日志，报如下错误：
161024 17:46:54 mysqld_safe Starting mysqld daemon with databases from /var/lib/mysql
161024 17:46:54 [Warning] Using unique option prefix key_buffer instead of key_buffer_size is deprecated and will be removed in a future release. Please use the full name instead.
161024 17:46:54 [Note] /usr/sbin/mysqld (mysqld 5.5.52-0ubuntu0.14.04.1) starting as process 12834 ...
161024 17:46:54 [Warning] Can't create test file /var/lib/mysql/host150.lower-test
161024 17:46:54 [Warning] Can't create test file /var/lib/mysql/host150.lower-test
161024 17:46:54 [Warning] Using unique option prefix myisam-recover instead of myisam-recover-options is deprecated and will be removed in a future release. Please use the full name instead.
161024 17:46:54 [Note] Plugin 'FEDERATED' is disabled.
/usr/sbin/mysqld: Can't find file: './mysql/plugin.frm' (errno: 13)
161024 17:46:54 [ERROR] Can't open the mysql.plugin table. Please run mysql_upgrade to create it.
161024 17:46:54 InnoDB: The InnoDB memory heap is disabled
161024 17:46:54 InnoDB: Mutexes and rw_locks use GCC atomic builtins
161024 17:46:54 InnoDB: Compressed tables use zlib 1.2.8
161024 17:46:54 InnoDB: Using Linux native AIO
161024 17:46:54 InnoDB: Initializing buffer pool, size = 128.0M
161024 17:46:54 InnoDB: Completed initialization of buffer pool
161024 17:46:54 InnoDB: Operating system error number 13 in a file operation.
InnoDB: The error means mysqld does not have the access rights to
InnoDB: the directory.
InnoDB: File name ./ibdata1
InnoDB: File operation call: 'open'.
InnoDB: Cannot continue operation.
161024 17:46:54 mysqld_safe mysqld from pid file /var/run/mysqld/mysqld.pid ended
原以为是系统目录权限的问题，修改了目录权限。
chown -R mysql:mysql /data/mysql
chmod -R 755 /data/mysql
再次尝试启动，还是不行。
最后搜索到一个解决办法：
/etc/init.d/apparmor stop
/etc/init.d/apparmor teardown
update-rc.d -f apparmor remove
apt-get purge apparmor
reboot
再次启动就ok了。
查了一下apparmor，是一个加入内核的巩固Linux内核安全的软件，卸载掉这个应该不是一个好的解决办法，如果大家有什么更好的办法，欢迎指点。
说明一下，我后来把/data/MySQL做为软链到了/var/lib目录。