正常是MySQL开机自动启动的，可是这次没有启动；
使用 service mysqld start 启动时提示：MySQL Daemon failed to start.
正在启动 mysqld： [失败]
查看MySQL日志
vi /var/log/mysqld.log
:$ 移动到文件尾，发现如下错误：
2016-07-02T01:34:44.454839Z 0 [ERROR] InnoDB: Write to file ./ibtmp1failed at offset 0, 1048576 bytes should have been written, only 0 were written. Operating system error number 28. Check that your OS and file system support files of this size. Check also that the disk is not full or a disk quota exceeded.
2016-07-02T01:34:44.454863Z 0 [ERROR] InnoDB: Error number 28 means 'No space left on device'
2016-07-02T01:34:44.454909Z 0 [Note] InnoDB: Some operating system error numbers are described at http://dev.mysql.com/doc/refman/5.7/en/operating-system-error-codes.html
2016-07-02T01:34:44.454925Z 0 [ERROR] InnoDB: Could not set the file size of './ibtmp1'. Probably out of disk space
2016-07-02T01:34:44.454936Z 0 [ERROR] InnoDB: Unable to create the shared innodb_temporary
2016-07-02T01:34:44.454948Z 0 [ERROR] InnoDB: Plugin initialization aborted with error Generic error
2016-07-02T01:34:45.056320Z 0 [Note] InnoDB: Removed temporary tablespace data file: "ibtmp1"
2016-07-02T01:34:45.056344Z 0 [ERROR] Plugin 'InnoDB' init function returned error.
2016-07-02T01:34:45.056354Z 0 [ERROR] Plugin 'InnoDB' registration as a STORAGE ENGINE failed.
2016-07-02T01:34:45.056364Z 0 [ERROR] Failed to initialize plugins.
2016-07-02T01:34:45.056372Z 0 [ERROR] Aborting
其中注意到：'No space left on device'，没有磁盘空间了；
删除了一部分没用的文件，然后再启动就可以了。
相关：
Navicat for MySQL出现1030-Got error 28 from storage engine错误
mysql无法启动ERROR! MySQL is running but PID file could not be found
