线上的一套MySQL 5.6的从库，因为OOM而导致数据库重启。
# tail -50 /var/log/messages
Nov 13 16:00:40 app-push-db2 kernel: Out of memory: Kill process 26121 (mysqld) score 957 or sacrifice child
Nov 13 16:00:40 app-push-db2 kernel: Killed process 26121 (mysqld) total-vm:18804176kB, anon-rss:15541596kB, file-rss:0kB, shmem-rss:0kB
报错原因：
这台实例所在的服务器上面部署了单实例MySQL，物理内存为16G，分配给数据库的缓存innodb_fuffer_pool_size为12G。
# free -g
total used free shared buff/cache available
Mem: 15 10 0 0 5 4
Swap: 0 0 0
为了提高性能，这台实例关闭了磁盘的swap交换。
# cat /etc/rc.local
#!/bin/bash
# THIS FILE IS ADDED FOR COMPATIBILITY PURPOSES
#
# It is highly advisable to create own systemd services or udev rules
# to run scripts during boot instead of using this file.
#
# In contrast to previous versions due to parallel execution during boot
# this script will NOT be run after all other services.
#
# Please note that you must run 'chmod +x /etc/rc.d/rc.local' to ensure
# that this script will be executed during boot.
touch /var/lock/subsys/local
swapoff -a
当客户端连接多的时候，导致内存资源耗尽。
解决方法：
将innodb_buffer_pool_size调整为10G，预留出一些内存给操作系统和客户端连接。
重启数据库，使参数生效。
# grep innodb_buffer_pool /etc/my.cnf
innodb_buffer_pool_size=10G
