为什么mysqld启动报错

在一台ubuntu测试机器上启动一个mysql实例，本来应该是一件很简单的事情，

启动的时候却报错了：

mysqld_safe --defaults-file=/etc/mysql/my3307.cnf
160218 15:45:13 mysqld_safe Can't log to error log and syslog at the same time.  Remove all --log-error configuration options for --syslog to take effect.
160218 15:45:13 mysqld_safe Logging to '/var/log/mysql/error3307.log'.
160218 15:45:13 mysqld_safe Starting mysqld daemon with databases from /var/lib/mysql3307
160218 15:45:15 mysqld_safe mysqld from pid file /var/run/mysqld/mysqld3307.pid ended

查看错误日志/var/log/mysql/error3307.log

发现是由于权限问题无法创建ibdata文件：

160218 12:19:09 InnoDB: Completed initialization of buffer pool
160218 12:19:09  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: 'create'.
InnoDB: Cannot continue operation.

查看目录，理论上应该有读写权限的啊：

ls -ld /var/lib/mysql3307
drwx------ 4 mysql mysql 4096 Feb 18 15:09 /var/lib/mysql3307

难道还有其它什么权限限制？

用dmesg看一下，确实有一些报错，

[4489099.208660] type=1400 audit(1455768562.142:57): apparmor="DENIED" operation="mknod" profile="/usr/sbin/mysqld" name="/var/lib/mysql3307/ibdata1" pid=19775 comm="mysqld" requested_mask="c" denied_mask="c" fsuid=105 ouid=105

查看/usr/sbin/mysqld的配置：

vi /etc/apparmor.d/usr.sbin.mysqld

增加mysql3307目录的权限，重启apparmor：

 /var/lib/mysql3307/ r,
 /var/lib/mysql3307/** rwk,

/etc/init.d/apparmor restart

再次重新启动mysqld，发现还是报错，不过这次报错的信息不一样了：

160218 12:44:03 [ERROR] Can't start server : Bind on unix socket: Permission denied
160218 12:44:03 [ERROR] Do you already have another mysqld server running on socket: /var/run/mysqld/mysqld3307.sock ?
160218 12:44:03 [ERROR] Aborting

dmesg也能看到错误消息：

[4491181.040673] type=1400 audit(1455770643.975:99): apparmor="DENIED" operation="mknod" profile="/usr/sbin/mysqld" name="/run/mysqld/mysqld3307.sock" pid=24093 comm="mysqld" requested_mask="c" denied_mask="c" fsuid=105 ouid=105

到/etc/apparmor.d/usr.sbin.mysqld增加配置，重启apparmor

 /var/run/mysqld/mysqld3307.pid rw,
 /var/run/mysqld/mysqld3307.sock w,
 /run/mysqld/mysqld3307.pid rw,
 /run/mysqld/mysqld3307.sock w,

再次重启mysqld，还是报错，不过这次是新的报错：

160218 12:57:27 [Note] Server socket created on IP: '0.0.0.0'.
160218 12:57:27 [ERROR] Fatal error: Can't open and lock privilege tables: Table 'mysql.host' doesn't exist
160218 12:57:27 mysqld_safe mysqld from pid file /var/run/mysqld/mysqld3307.pid ended

跑一下mysql_install_db --defaults-file=/etc/mysql/my3307.cnf

这次mysqld终于可以正常启动了：

160218 14:44:59 [Note] Server socket created on IP: '0.0.0.0'.
160218 14:44:59 [Note] Event Scheduler: Loaded 0 events
160218 14:44:59 [Note] /usr/sbin/mysqld: ready for connections.
Version: '5.5.46-0ubuntu0.14.04.2'  socket: '/var/run/mysqld/mysqld3307.sock'  port: 3307  (Ubuntu)