問題：mysql 啟動報錯

[[email protected] ~]# service mysqld restart
MySQL server PID file could not be found!                  [FAILED]
Starting MySQL.Logging to '/var/log/mysql/error.log'.
The server quit without updating PID file (/data/mysql/iZuz[FAILED]2iZ.pid).

第一直覺告訴我google “ MySQL server PID file could not be found! [FAILED]”，可基本沒什麼用。
後開有人說看看報錯資訊，於是：

[[email protected] ~]# tail -n 50 /data/mysql/iZuzc9f4ma6h2iZ.err
InnoDB: mmap(137363456 bytes) failed; errno 12
2017-12-28 17:09:44 25065 [ERROR] InnoDB: Cannot allocate memory for the buffer pool
2017-12-28 17:09:44 25065 [ERROR] Plugin 'InnoDB' init function returned error.
2017-12-28 17:09:44 25065 [ERROR
 
] Plugin 'InnoDB' registration as a STORAGE ENGINE failed.
2017-12-28 17:09:44 25065 [ERROR] Unknown/unsupported storage engine: InnoDB
2017-12-28 17:09:44 25065 [ERROR] Aborting

• Out of memory error: InnoDB:
  Fatal error: cannot allocate memory for the buffer pool
• InnoDB can’t start without enough memory
  [ERROR] Plugin ‘InnoDB’ init function returned error.
  [ERROR] Plugin ‘InnoDB’ registration as a STORAGE ENGINE failed.
  [ERROR] Unknown/unsupported storage engine: InnoDB
  [ERROR] Aborting
• mysqld is shutting down, which in this context, really means failing to restart!
  [Note] /usr/sbin/mysqld: Shutdown complete

ok，明白了嗎？
於是繼續，來看看記憶體佔用情況：

[[email protected] ~]# free -m
              total        used        free      shared  buff/cache   available
Mem:            992         435         413           1         142         403
Swap:             0           0           0

顯然，結果很磕饞。記憶體僅413M，而且沒有 Swap 分割槽。

那麼問題來了，mysql 啟動到底需要多少記憶體？

官網是這什麼回答的：

MySQL allocates buffers and caches to improve performance of database operations. The default configuration is designed to permit a MySQL server to start on a virtual machine that has approximately512MB of RAM. You can improve MySQL performance by increasing the values of certain cache and buffer-related system variables. You can also modify the default configuration to run MySQL on systems with limited memory.

於是，那麼有什麼辦法可以小記憶體啟動 mysql 嗎？

答案是有，

vim /etc/my.cnf

新增：

performance_schema_max_table_instances=200
table_definition_cache=200
table_open_cache=128

ok，再次執行 service mysqld restart ，沒什麼問題。