默认情况下MySQL数据库放在/var/lib/mysql
因为磁盘空间问题，我们可能需要迁移这个目录。通常的想法是
service mysql stop
mv /var/lib/mysql /mnt/data/
ln -s /mnt/data/mysql /var/lib/mysql
service mysql start
然而，会发现mysql服务无法启动，/var/log/mysql/error.log会报以下错误：
2018-11-20T01:52:17.406707Z 0 [Warning] Can't create test file /mnt/data/mysql/uservdc.lower-test
2018-11-20T01:52:17.406751Z 0 [Note] /usr/sbin/mysqld (mysqld 5.7.24-0ubuntu0.18.04.1) starting as process 9538 ...
2018-11-20T01:52:17.409857Z 0 [Warning] Can't create test file /mnt/data/mysql/uservdc.lower-test
2018-11-20T01:52:17.409881Z 0 [Warning] Can't create test file /mnt/data/mysql/uservdc.lower-test
2018-11-20T01:52:17.412977Z 0 [Note] InnoDB: PUNCH HOLE support available
2018-11-20T01:52:17.413003Z 0 [Note] InnoDB: Mutexes and rw_locks use GCC atomic builtins
2018-11-20T01:52:17.413008Z 0 [Note] InnoDB: Uses event mutexes
2018-11-20T01:52:17.413013Z 0 [Note] InnoDB: GCC builtin __atomic_thread_fence() is used for memory barrier
2018-11-20T01:52:17.413017Z 0 [Note] InnoDB: Compressed tables use zlib 1.2.11
2018-11-20T01:52:17.413021Z 0 [Note] InnoDB: Using Linux native AIO
2018-11-20T01:52:17.413289Z 0 [Note] InnoDB: Number of pools: 1
2018-11-20T01:52:17.413397Z 0 [Note] InnoDB: Using CPU crc32 instructions
2018-11-20T01:52:17.415067Z 0 [Note] InnoDB: Initializing buffer pool, total size = 128M, instances = 1, chunk size = 128M
2018-11-20T01:52:17.424103Z 0 [Note] InnoDB: Completed initialization of buffer pool
2018-11-20T01:52:17.426401Z 0 [Note] InnoDB: If the mysqld execution user is authorized, page cleaner thread priority can be changed. See the man page of setpriority().
2018-11-20T01:52:17.436459Z 0 [ERROR] InnoDB: The innodb_system data file 'ibdata1' must be writable
2018-11-20T01:52:17.436489Z 0 [ERROR] InnoDB: The innodb_system data file 'ibdata1' must be writable
2018-11-20T01:52:17.436513Z 0 [ERROR] InnoDB: Plugin initialization aborted with error Generic error
2018-11-20T01:52:18.037230Z 0 [ERROR] Plugin 'InnoDB' init function returned error.
2018-11-20T01:52:18.037273Z 0 [ERROR] Plugin 'InnoDB' registration as a STORAGE ENGINE failed.
2018-11-20T01:52:18.037282Z 0 [ERROR] Failed to initialize builtin plugins.
2018-11-20T01:52:18.037288Z 0 [ERROR] Aborting
可是我们明明是有权限的，问题在于一个叫AppArmor的软件。我们需要在下面文件添加2行，/etc/apparmor.d/local/usr.sbin.mysqld:
/mnt/data/ r, /mnt/data/** rwk,
然后重启apparmor服务，启动mysql：
service apparmor reload
service mysql start
参考：https://askubuntu.com/questions/758898/mysql-wont-start-after-changing-the-datadir-14-04-mysql-5-7
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