mysql数据库报错 无法更新PID文件

在ECS实例上运行了博客网站上面有 tomcat  redis  nginx  mysql  但是内存有限，导致内核进程终结者 终结掉了mysql进程，然后启动mysql 就开始报错，无法启动

经过查找资料，发现有很多初级的解决办法，例如删除/etc/my.cnf   初始化数据库  指定datadir路径等等。都是复制来复制去的 千篇一律
首先查看日志吧：

2016-07-04 09:55:55 0 [Note] /usr/local/mysql/bin/mysqld (mysqld 5.6.28) starting as process 24460 ...
2016-07-04 09:55:55 24460 [Note] Plugin 'FEDERATED' is disabled.
2016-07-04 09:55:55 24460 [Note] InnoDB: Using atomics to ref count buffer pool pages
2016-07-04 09:55:55 24460 [Note] InnoDB: The InnoDB memory heap is disabled
2016-07-04 09:55:55 24460 [Note] InnoDB: Mutexes and rw_locks use GCC atomic builtins
2016-07-04 09:55:55 24460 [Note] InnoDB: Memory barrier is not used
2016-07-04 09:55:55 24460 [Note] InnoDB: Compressed tables use zlib 1.2.3
2016-07-04 09:55:55 24460 [Note] InnoDB: Using CPU crc32 instructions
2016-07-04 09:55:55 24460 [Note] InnoDB: Initializing buffer pool, size = 128.0M
InnoDB: mmap(137363456 bytes) failed; errno 12
2016-07-04 09:55:55 24460 [ERROR] InnoDB: Cannot allocate memory for the buffer pool
2016-07-04 09:55:55 24460 [ERROR] Plugin 'InnoDB' init function returned error.
2016-07-04 09:55:55 24460 [ERROR] Plugin 'InnoDB' registration as a STORAGE ENGINE failed.
2016-07-04 09:55:55 24460 [ERROR] Unknown/unsupported storage engine: InnoDB
2016-07-04 09:55:55 24460 [ERROR] Aborting

排查办法：

[root@dfd data]# free -m
total used free shared buffers cached
Mem: 994 597 397 2 5 40
-/+ buffers/cache: 551 443
Swap: 0 0 0

解决办法：
修改my.cnf
innodb_buffer_pool_size=28M即可

Buffuers不足初始化失败！