Redis的LRU机制介绍
在Redis中，如果设置的maxmemory，那就要配置key的回收机制参数maxmemory-policy，默认volatile-lru，参阅Redis作者的原博客：antirezweblog>>RedisasanLRUcache
原文中写得很清楚：
AnotherwaytouseRedisasacacheisthemaxmemorydirective,afeaturethatallowsspecifyingamaximumamountofmemorytouse.Whennewdataisaddedtotheserver,andthememorylimitwasalreadyreached,theserverwillremovesomeolddatadeletingavolatilekey,thatis,akeywithanEXPIRE(atimeout)set,evenifthekeyisstillfarfromexpiringautomatically.
在Redis服务器占用内存达到maxmemory的情况下，当再想增加内存占用时，会按maxmemory-policy机制将老的数据删除。这里简单说一下volatile-lru，Redis会按LRU算法删除设置了过期时间但还没有过期的key，而对于没有设置过期时间的key，Redis是永远保留的。当然，如果你不想删除没有过期的key，那可以使用noeviction机制
#MAXMEMORYPOLICY:howRediswillselectwhattoremovewhenmaxmemory #isreached?Youcanselectamongfivebehavior: # #volatile-lru->removethekeywithanexpiresetusinganLRUalgorithm #allkeys-lru->removeanykeyaccordinglytotheLRUalgorithm #volatile-random->removearandomkeywithanexpireset #allkeys-random->removearandomkey,anykey #volatile-ttl->removethekeywiththenearestexpiretime(minorTTL) #noeviction->don'texpireatall,justreturnanerroronwriteoperations