Question

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的耗尽？它如何决定要删除哪些数据或将哪些数据保留在内存中？

Answer 1

如果您打开了虚拟内存功能（我认为是2.0或2.2版本中的新功能），那么当内存耗尽时，Redis会开始将“不那么频繁使用”的数据存储到磁盘上。

如果Redis中的虚拟内存被禁用，则看起来好像操作系统的虚拟内存开始耗尽（即交换），性能大幅下降。

现在，您还可以使用maxmemory参数配置Redis，以防止Redis使用更多内存（默认设置）。

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集，试图删除最近未使用过的密钥。
volatile-ttl删除其中的一个键那些有过期集，试图短暂的剩余时间删除键生活。
volatile-random删除a 带有随机密钥的随机密钥到期集。
allkeys-lru喜欢 volatile-lru，但会删除所有钥匙，普通钥匙或钥匙过期集。
allkeys随机像volatile-random，但会删除各种键，都是普通键和过期设置的密钥。

如果您选择仅删除设置了EXPIRE的键的策略，那么当Redis内存不足时，程序似乎只是中止malloc（）操作。也就是说，如果您尝试存储更多数据，那么操作就会失败。

更多信息的一些链接（因为你不应该只是接受我的话）：

Answer 2

来自redis.conf，版本2.8

# 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

Answer 3

我刚刚开始阅读关于Redis的内容，所以我不是正面的。但是，我确实遇到了一些可能有用的花絮。

以下是http://antirez.com/post/redis-as-LRU-cache.html的摘录：

将Redis用作缓存的另一种方法是 maxmemory指令，一个功能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和内存中已达到限制，服务器将删除一些旧数据删除 volatile键，即带键的键 EXPIRE（超时）设置，即使是关键还远未到期自动。

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

Answer 4

如果你想知道当Redis（2.8）达到其配置定义的最大值时实际响应的是什么，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'.

Answer 5

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

Answer 6

我最近经历了一个无内存的情况，我的应用程序停止运行（写入不可能，读取成为可能），运行PHP脚本在中途停止运行并且必须为kill -9手动（即使在内存可用后）。

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？备份是你的朋友。

Answer 7

Redis不是像memcached这样的缓存，默认情况下（maxmemory-policy参数设置为noeviction）所有放入redis的数据都不会被删除，唯一的例外是使用EXPIRE。

当内存耗尽时，Redis会做什么？

7 个答案: