redis缓存策略解析

# MAXMEMORY POLICY: how Redis will select what to remove when maxmemory

# is reached. You can select among five behaviors:

#

# volatile-lru -> Evict using approximated LRU among the keys with an expire set.

 最近最少使用，使用LRU算法移除key,只对设置了过期时间的键

# allkeys-lru -> Evict any key using approximated LRU. 

使用LRU算法移除key

# volatile-lfu -> Evict using approximated LFU among the keys with an expire set.

# allkeys-lfu -> Evict any key using approximated LFU.

# volatile-random -> Remove a random key among the ones with an expire set.

在过期的集合中移除随机的key,只对设置了过期时间的键

# allkeys-random -> Remove a random key, any key.

移除随机的key

# volatile-ttl -> Remove the key with the nearest expire time (minor TTL)

移除那些TTL值最小的Key,即那些最近要过期的key

# noeviction -> Don't evict anything, just return an error on write operations.

 不进行移除，针对写操作，只是返回错误信息

#

# LRU means Least Recently Used

# LFU means Least Frequently Used

#

# Both LRU, LFU and volatile-ttl are implemented using approximated

# randomized algorithms.

持久化：rdb  aof

Rdb：在指定的时间间隔内将内存中的数据集快照写入磁盘

 snapshot快照，恢复时是将快照文件直接读到内存中

优点：适合大规模的数据恢复

     对数据完整性和一致性要求不高

劣势：

    在一定时间间隔做一次备份，所以如果redis意外down掉的话，就会丢失最后一次快照后的所有修改

   fork的时候，内存中的数据被克隆了一份，大致2杯的膨胀性需要考虑