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问：MySQL中有2000w数据，Redis中只存20w的数据，如何保证Redis中的数据都是热点数据？
答：相关知识：Redis内存数据集大小上升到一定大小（设置项为maxmemory）的时候，就会执行数据淘汰策略（回收策略）。Redis提供了6种数据淘汰策略。
Redis配置文件中的关于设置maxmemory（Redis的最大使用内存）的代码maxmemory <bytes>，默认是0并且以字节为单位的。如果有新的数据添加，超过最大内存，则会使Redis崩溃。Redis内存数据集大小上升到一定大小（设置项为maxmemory）的时候，就会执行数据淘汰策略（回收策略）Redis支持6种数据淘汰策略，默认是maxmemory-policy noeviction：禁止驱逐。以下是6种数据淘汰算法的介绍：
- volatile-lru：使用LRU算法删除具有过期时间的密钥
- allkeys-lru：根据LRU算法删除任何密钥
- volatile-random：删除带有过期集的随机密钥
- allkeys-random：删除一个随机密钥
- volatile-ttl：删除具有最近过期时间的密钥
- noeviction：完全不过期，只是在写操作上返回一个错误
源文件相关配置代码（不同的版本可能有差异，重要的说明项已在原文代码中提供翻译）:
# Note on units: when memory size is needed, it is possible to specify
# it in the usual form of 1k 5GB 4M and so forth:
#
# 1k => 1000 bytes
# 1kb => 1024 bytes
# 1m => 1000000 bytes
# 1mb => 1024*1024 bytes
# 1g => 1000000000 bytes
# 1gb => 1024*1024*1024 bytes
# 单位是不区分大小写的，所以1GB 1Gb 1gB都是一样的
# units are case insensitive so 1GB 1Gb 1gB are all the same.
# WARNING: not setting maxmemory will cause Redis to terminate with an
# out-of-memory exception if the heap limit is reached.
# 警告:未设置maxmemory将导致Redis在达到堆限制时发生内存不足异常
# maxmemory <bytes>
# 设置最大内存，默认单位是byte，即字节
# 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:当达到最大内存时，Redis如何选择删除什么。您可以选择五种行为:
#
# volatile-lru -> remove the key with an expire set using an LRU algorithm :使用LRU算法删除具有过期集的密钥
# allkeys-lru -> remove any key according to the LRU algorithm :根据LRU算法删除任何密钥
# volatile-random -> remove a random key with an expire set ：删除带有过期集的随机密钥
# allkeys-random -> remove a random key, any key :删除一个随机密钥，任何密钥
# volatile-ttl -> remove the key with the nearest expire time (minor TTL) :删除具有最近过期时间的密钥
# noeviction -> don't expire at all, just return an error on write operations :完全不过期，只是在写操作上返回一个错误
#
# Note: with any of the above policies, Redis will return an error on write
# operations, when there are no suitable keys for eviction.
# 注意:对于以上任何一种策略，当没有合适的清除键时，Redis将在写操作上返回一个错误。
# 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:
# 默认的是:最大内存策略:不清除
# maxmemory-policy noeviction
# 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:当达到最大内存时，Redis将如何选择删除什么。您可以选择五种行为
# LRU 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 a bit more CPU. 3 is very fast but not very accurate.
#
# maxmemory-samples 5