Redis持久化RDB模式，Redis借助了fork命令的copy on write机制。在生成快照时，将当前进程整个复制出来，fork出一个子进程，然后
Redis持久化RDB模式的工作原理：
Redis持久化RDB模式，Redis借助了fork命令的copy on write机制。在生成快照时，将当前进程整个复制出来，fork出一个子进程，然后在子进程中循环所有的数据，将数据写成为RDB文件。
Redis持久化RDB模式引发的问题：
RDB模式需要Redis服务所占内存的1倍的内存
例如一台机器总共16G内存，用了10G内存做Redis服务，假如这10G内存都占满了
这时运行save命令，这时会把10G的进程再复制一遍，变成20G，超过了16G就产生交换，，
如果虚拟内存设置了为4G，这样save也能完成，由于引发大量的交换，会很慢；
如果虚拟内存设置了小于4G，这时Redis就会崩溃，而且数据也不会完整的保存到快照文件中，
重新启动，就会发现数据已经丢失了很多。
Ubuntu 14.04下Redis安装及简单测试
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Redis 的详细介绍：请点这里
Redis 的下载地址：请点这里
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