我可能完全不理解，但我对缓存存储在开始添加持久性功能之前的工作方式的理解是，项目会根据其 TTL 过期。 .如果商店开始填满可用的 RAM，他们每个人都会有自己的算法来过期商店中最不“重要”的键。

现在我读到 Redis 有 persistence特征。但您可以将它们关闭。假设您关闭持久性，当 RAM 填满时会发生什么？ Redis 如何决定什么要过期？

我希望有大量没有 TTL 的数据，并希望确保让 Redis 确定哪些内容会过期是安全的。

最佳答案

我不认为这个问题与虚拟内存管理有关，而是与 Redis 中项目的过期有关，这是一个完全不同的话题。

与memcached相反，Redis不仅仅是一个缓存。因此，用户应该使用各种机制来选择项目驱逐策略。您可以驱逐所有元素，也可以只驱逐其中的一部分。

应该在配置文件中使用 maxmemory 和 maxmemory-policy 参数选择通用策略，如下所述:

# Don't use more memory than the specified amount of bytes.
# When the memory limit is reached Redis will try to remove keys with an
# EXPIRE set. It will try to start freeing keys that are going to expire
# in little time and preserve keys with a longer time to live.
# Redis will also try to remove objects from free lists if possible.
#
# If all this fails, Redis will start to reply with errors to commands
# that will use more memory, like SET, LPUSH, and so on, and will continue
# to reply to most read-only commands like GET.
#
# WARNING: maxmemory can be a good idea mainly if you want to use Redis as a
# 'state' server or cache, not as a real DB. When Redis is used as a real
# database the memory usage will grow over the weeks, it will be obvious if
# it is going to use too much memory in the long run, and you'll have the time
# to upgrade. With maxmemory after the limit is reached you'll start to get
# errors for write operations, and this may even lead to DB inconsistency.
#
maxmemory <bytes>

# MAXMEMORY POLICY: how Redis will select what to remove when maxmemory
# is reached? You can select among five behavior:
#
# volatile-lru -> remove the key with an expire set using an LRU algorithm
# allkeys-lru -> remove any key accordingly to the LRU algorithm
# volatile-random -> remove a random key with an expire set
# allkeys->random -> remove a random key, any key
# volatile-ttl -> remove the key with the nearest expire time (minor TTL)
# noeviction -> don't expire at all, just return an error on write operations
#
# Note: with all the kind of policies, Redis will return an error on write
#       operations, when there are not suitable keys for eviction.
#
#       At the date of writing this 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
#
# The default is:
#
maxmemory-policy volatile-lru

# LRU and minimal TTL algorithms are not precise algorithms but approximated
# algorithms (in order to save memory), so you can select as well the sample
# size to check. For instance for default Redis will check three keys and
# pick the one that was used less recently, you can change the sample size
# using the following configuration directive.
#
maxmemory-samples 3

然后可以使用以下命令设置单个项目的过期时间: EXPIRE EXPIREAT 每项过期属性对于 volatile-* 策略很有用。 过期也可以使用 PERSIST 删除.

expiration 属性会增加一些内存开销，因此只有在需要时才应使用它。

最后，值得一提的是，对象的一部分不能过期，只能是整个对象本身。例如，一个键对应的整个列表或集合可以过期，但单个列表或集合项不能。

关于caching - 当 RAM 开始填满时，Redis 如何工作？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/8652388/