Redis 内存不足时会做啥？

Posted 2023-02-16

【中文标题】Redis 内存不足时会做啥？

【英文标题】：What does Redis do when it runs out of memory?Redis 内存不足时会做什么？

【发布时间】：2011-07-01 09:50:50

【问题描述】：

这可能是一个简单的问题，但我很难找到答案。 Redis 2.0 如何处理最大分配内存不足的问题？它如何决定删除哪些数据或将哪些数据保留在内存中？

【问题讨论】：

类似：***.com/questions/2604831/…

【参考方案1】：

Redis不像memcached那样是缓存，默认情况下（maxmemory-policy参数设置为noeviction）你放入redis的所有数据都不会被删除，唯一的例外是使用EXPIRE。

【讨论】：

那么当内存耗尽时它会怎么做呢？它只会将新数据存储在磁盘而不是内存中？

这（现在）不正确，Redis 有一个密钥驱逐机制，有几个可用的策略：redis.io/topics/lru-cache

@LoicAG：对我来说听起来完全正确......除非有过期政策，否则 Redis 不会驱逐任何密钥。这很好：例如，我不能让 Redis 自己摆脱密钥。

@Cory：如果设置了驱逐策略，它将删除现有密钥。但是，如果您没有设置任何驱逐政策，您应该会收到内存不足错误。

@Michael 我想这是一个术语问题：总是有一个 maxmemory-policy ，默认值确实是“noeviction”；但是“allkeys-lru”和“allkeys-random”策略会从整个集合中逐出键，而其他（“volatile-*”）策略会从定义了 TTL 的键子集中逐出键。

【参考方案2】：

我最近才开始阅读有关 Redis 的信息，所以我并不积极。但是，我确实遇到了一些可能有用的花絮。

这是来自http://antirez.com/post/redis-as-LRU-cache.html的sn-p：

使用 Redis 作为缓存的另一种方法是 maxmemory 指令，一个特性 允许指定最大值 要使用的内存量。当新数据 被添加到服务器，并且内存 已经达到限制，服务器 将删除一些旧数据删除一个 易失性密钥，即具有 EXPIRE（超时）设置，即使 密钥还远未到期 自动。

此外，Redis 2.0 有一个 VM 模式，其中所有键都必须适合内存，但很少使用的键的值可以在磁盘上：

http://redis.io/topics/virtual-memory http://antirez.com/post/redis-virtual-memory-story.html

【参考方案3】：

如果您启用了虚拟内存功能（编辑：现已弃用），那么当内存耗尽时，Redis 就会开始将“不那么常​​用”的数据存储到磁盘中。 p>

如果 Redis 中的虚拟内存被禁用（默认）并且设置了maxmemory 参数（默认），Redis 将不会使用超过maxmemory 允许的内存。如果关闭maxmemory，Redis 将开始使用虚拟内存（即交换），性能将大幅下降。

当达到maxmemory 时，较新版本的 Redis 有各种策略：

volatile-lru - 删除其中的一个键 那些有过期设置的，试图 删除最近未使用的密钥。 volatile-ttl - 删除其中的一个键 那些有过期设置的，试图 删除剩余时间短的密钥 生活。 volatile-random - 删除一个 随机密钥 过期设置。 allkeys-lru - 喜欢 volatile-lru，但会删除每个 一种钥匙，普通钥匙或钥匙 设置了过期时间。 allkeys-random - 喜欢volatile-random，但会删除 各种键，都是普通键 以及设置了过期时间的密钥。

如果你选择一个只删除带有 EXPIRE 集的键的策略，那么当 Redis 内存不足时，看起来程序只是中止了 malloc() 操作。也就是说，如果您尝试存储更多数据，写入操作就会失败。

更多信息的链接：

http://antirez.com/post/redis-as-LRU-cache.html http://eli.thegreenplace.net/2009/10/30/handling-out-of-memory-conditions-in-c/

【讨论】：

Redis 虚拟内存现已弃用。见redis.io/topics/virtual-memory

当您将 Redis 用作可回收缓存时，您会怎么做？在销毁之前，您如何管理将重要数据从 Redis 中取出并放入另一种形式的存储中？当内存填满时，你能把它和驱逐配对吗？

@jocull - 在数据填满之前将数据移出 Redis 是留给应用程序的一个细节。如果什么都不做，Redis 就会耗尽内存。此时，后续写入将失败。您可以监控 Redis 内存使用情况并编写自定义应用程序逻辑来移动数据。

【参考方案4】：

我最近遇到了没有可用内存的情况，我的应用程序停止了（无法写入，可以读取），正在运行的 php 脚本中途停止运行，必须kill -9'd手动（即使在内存可用之后）。

我假设发生了数据丢失（或数据不一致），因此我执行了flushdb 并从备份中恢复。学过的知识？备份是您的朋友。

【参考方案5】：

来自redis.conf，2.8 版

# Don't use more memory than the specified amount of bytes.
# When the memory limit is reached Redis will try to remove keys
# according to the eviction policy selected (see maxmemory-policy).
#
# If Redis can't remove keys according to the policy, or if the policy is
# set to 'noeviction', Redis will start to reply with errors to commands
# that would use more memory, like SET, LPUSH, and so on, and will continue
# to reply to read-only commands like GET.
#
# This option is usually useful when using Redis as an LRU cache, or to set
# a hard memory limit for an instance (using the 'noeviction' policy).
#
# WARNING: If you have slaves attached to an instance with maxmemory on,
# the size of the output buffers needed to feed the slaves are subtracted
# from the used memory count, so that network problems / resyncs will
# 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>

# MAXMEMORY POLICY: how Redis will select what to remove when maxmemory
# is reached. You can select among five behaviors:
#
# volatile-lru -> remove the key with an expire set using an LRU algorithm
# allkeys-lru -> remove any key according 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 any of the above policies, Redis will return an error on write
#       operations, when there are no suitable keys for eviction.
#
#       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
#
# The default is:
#
# maxmemory-policy volatile-lru

【讨论】：

Redis 3.2 中默认的maxmemory-policy 现在是noeviction: raw.githubusercontent.com/antirez/redis/3.2/redis.conf

【参考方案6】：

如果您想知道 Redis (2.8) 在达到其配置定义的最大值时实际响应什么，它看起来像这样：

$ redis-cli
127.0.0.1:6379> GET 5
"bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb"
127.0.0.1:6379> SET 5 a
(error) OOM command not allowed when used memory > 'maxmemory'.

【参考方案7】：

更新 redis 4.0

127.0.0.1:6379> MEMORY HELP
1) "MEMORY DOCTOR                        - Outputs memory problems report"
2) "MEMORY USAGE <key> [SAMPLES <count>] - Estimate memory usage of key"
3) "MEMORY STATS                         - Show memory usage details"
4) "MEMORY PURGE                         - Ask the allocator to release memory"
5) "MEMORY MALLOC-STATS                  - Show allocator internal stats"

/usr/local/etc/redis.conf

############################## MEMORY MANAGEMENT ################################

# Set a memory usage limit to the specified amount of bytes.
# When the memory limit is reached Redis will try to remove keys
# according to the eviction policy selected (see maxmemory-policy).
#
# If Redis can't remove keys according to the policy, or if the policy is
# set to 'noeviction', Redis will start to reply with errors to commands
# that would use more memory, like SET, LPUSH, and so on, and will continue
# to reply to read-only commands like GET.
#
# This option is usually useful when using Redis as an LRU or LFU cache, or to
# set a hard memory limit for an instance (using the 'noeviction' policy).
#
# WARNING: If you have slaves attached to an instance with maxmemory on,
# the size of the output buffers needed to feed the slaves are subtracted
# from the used memory count, so that network problems / resyncs will
# 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>

# 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.
# allkeys-lru -> Evict any key using approximated LRU.
# 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.
# allkeys-random -> Remove a random key, any key.
# volatile-ttl -> Remove the key with the nearest expire time (minor TTL)
# noeviction -> Don't evict anything, just return an error on write operations.
#
# LRU means Least Recently Used
# LFU means Least Frequently Used
#
# Both LRU, LFU and volatile-ttl are implemented using approximated
# randomized algorithms.
#
# Note: with any of the above policies, Redis will return an error on write
#       operations, when there are no suitable keys for eviction.
#
#       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
#
# The default is:
#
# maxmemory-policy noeviction

# LRU, LFU and minimal TTL algorithms are not precise algorithms but approximated
# algorithms (in order to save memory), so you can tune it for speed or
# accuracy. For default Redis will check five keys and pick the one that was
# used less recently, you can change the sample size using the following
# configuration directive.
#
# The default of 5 produces good enough results. 10 Approximates very closely
# true LRU but costs more CPU. 3 is faster but not very accurate.
#
# maxmemory-samples 5