这是一个真实的案例，在2009年，2010年，主机多次被Hang死，当时的情况是主机可以ping通，但ssh却连接不上，导致cluster无法切换，只能人工去重启那台被Hang死的
这是一个真实的案例，在2009年，2010年，主机多次被Hang死，当时的情况是主机可以ping通，但ssh却连接不上，导致cluster无法切换，只能人工去重启那台被Hang死的机器，才能正常切换。到底是什么原因引起的呢，在看了淘宝这篇文章后，
茅塞顿开，我们的主机是72G内存，48G是MySQL（Innodb_Buffer_Pool）内存缓冲池所用，但在机器正常的情况 下，free -m，发现内存已经超过了48G，这是因为其他程序或者系统模块又需要额外的内存，比如cp一个大文件，或者在主库上mysqldump数据库等等，这时 当物理内存不够用的时候，操作系统就会把MySQL所拥有的一部分地址空间映射到swap上去。而我们当时，没有建立swap分区是考虑到将分配给 MySQL的地址空间映射到swap上，导致性能会有突起波动，但这是一把双刃剑，香港服务器租用，虽然避免了使用swap分区，但结果会导致机器被Hang死。
所以就这个问题，我们后来的解决方案是：
一、增加2G SWAP分区，避免内存耗尽时，香港服务器，机器不会死机，可以给予些缓冲，重启前端程序释放MySQL压力。
二、增加内存监控，当内存使用率达到90%，通过重启MYSQL来释放内存，避免机器死机。
三、开发设计的前端CACHE层，减少了数据库层的压力。
在2011年至2012年10月，机器都运行稳定，虚拟主机，没有出现过宕机。
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