浅谈redis的maxmemory设置以及淘汰策略

浅谈浅谈redis的的maxmemory设置以及淘汰策略设置以及淘汰策略

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redis的maxmemory参数用于控制redis可使用的最大内存容量。如果超过maxmemory的值，就会动用淘汰策略来处理expaire字典中的键。

关于关于redis的淘汰策略：的淘汰策略：

Redis提供了下面几种淘汰策略供用户选择，其中默认的策略为noeviction策略：

· noeviction：当内存使用达到阈值的时候，所有引起申请内存的命令会报错。

· allkeys-lru：在主键空间中，优先移除最近未使用的key。

· volatile-lru：在设置了过期时间的键空间中，优先移除最近未使用的key。

但是在集群环境下在集群环境下（尤其是有多个slavers的情形），maxmeomory的值并不是实际redis使用的内存，这个选项值并没有包括slaver的output buffer。

redis早期版本出过一个bug，在多个slaver的情形下，设置了maxmemory值，同时设定了淘汰策略，会造成master上的数据被渐渐擦除。

antirez先生给出了这个问题的原因：

The issue happens for the following reason:

Redis reached the configured limit, so it tries to expire keys.

Evicting keys turns into explicit DELs sent to slaves, since masters control the eviction of slaves for well known reasons.

But this way if there are enough slaves, emitting the protocol in the output buffers will actually take more memory than the amount freed removing keys...

So the key eviction process starts to enter into an infinite loop.

Up to a given point the fact that there is a static buffer part in the output queue of every client (including slaves) mitigate this in certain conditions, but once Redis can't use the output buffer but must use the queue of objects the infinite loop is triggered.

# not trigger a loop where keys are evicted, and in turn the output

# buffer of slaves is full with DELs of keys evicted triggering the deletion

# of more keys, and so forth until the database is completely emptied.

#

# In short... if you have slaves attached it is suggested that you set a lower

# limit for maxmemory so that there is some free RAM on the system for slave

# output buffers (but this is not needed if the policy is 'noeviction').

#

# maxmemory <bytes></bytes>

由此可见，如果有slaver的情况下，建议适当调低maxmemory，给output buffer留出一定的可用空间是合理的。