Redis过期策略以及内存淘汰机制

# MAXMEMORY POLICY: how Redis will select what to remove when maxmemory
# is reached. You can select among five behaviors:
#最大内存策略：当到达最大使用内存时，你可以在下面5种行为中选择，Redis如何选择淘汰数据库键

#当内存不足以容纳新写入数据时

# volatile-lru -> remove the key with an expire set using an LRU algorithm
# volatile-lru ：在设置了过期时间的键空间中，移除最近最少使用的key。这种情况一般是把 redis 既当缓存，又做持久化存储的时候才用。

# allkeys-lru -> remove any key according to the LRU algorithm
# allkeys-lru ：移除最近最少使用的key （推荐）

# volatile-random -> remove a random key with an expire set
# volatile-random ：在设置了过期时间的键空间中，随机移除一个键，不推荐

# allkeys-random -> remove a random key, any key
# allkeys-random ：直接在键空间中随机移除一个键，弄啥叻

# volatile-ttl -> remove the key with the nearest expire time (minor TTL)
# volatile-ttl ：在设置了过期时间的键空间中，有更早过期时间的key优先移除 不推荐

# noeviction -> don't expire at all, just return an error on write operations
# noeviction ：不做过键处理，只返回一个写操作错误。不推荐

# Note: with any of the above policies, Redis will return an error on write
# operations, when there are no suitable keys for eviction.
# 上面所有的策略下，在没有合适的淘汰删除的键时，执行写操作时，Redis 会返回一个错误。下面是写入命令：
# At the date of writing these commands are: set setnx setex append
# incr decr rpush lpush rpushx lpushx linsert lset rpoplpush sadd
# sinter sinterstore sunion sunionstore sdiff sdiffstore zadd zincrby
# zunionstore zinterstore hset hsetnx hmset hincrby incrby decrby
# getset mset msetnx exec sort

# 过期策略默认是：
# The default is:
# maxmemory-policy noeviction