Redis学习——Redis持久化之AOF备份方式保存数据

新技术的出现一定是在老技术的基础之上，并且完善了老技术的某一些不足的地方，新技术和老技术就如同JAVA中的继承关系。子类（新技术）比父类（老技术）更加的强大！

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在前面介绍了Redis学习——Redis持久化之RDB备份方式保存数据之后，下面在整理和学习一下Redis的AOF方式保存数据。

首先抛出几个问题，然后回答这些问题，最后逐步的对AOF进行介绍。

1.RDB可以搞定备份恢复的事情，为什么还会出现AOF？

	答案：这个问题的答案可以参看，上面说的Redis持久化之RDB备份方式保存数据中的RDB的缺点。
	也就是使用RDB进行保存时候，如果Redis服务器发送故障，那么会丢失最后一次备份的数据！
	AOF出现试着来解决这个问题！

2.同时出现RDB和AOF是冲突呢？还是协作？

答案：是协作，不会冲突!那么是如何协作，首先加载哪一个文件呢？
进行测试，生成dump.rdb和appendonly.aof文件，然后在appendonly.aof使文件最后随便加入一些东西，使文件出错，然后重新启动redis服务，发现服务没有启动成功！那么就可以知道首先加载的是aof文件，使用redis-check-aof 工具修复aof文件，重新启动，发现启动成功！
总结：两者可以共存，但是首先启动找的是aof。

如何修复：redis-check-aof --fix appendonly.aof

3.AOF有什么优缺点？

这个看文章下面的介绍吧！

一：Redis的AOF是什么？

官网-AOF中文：http://www.redis.cn/topics/persistence.html

以日志的形式来记录每个写操作（读操作不记录），将Redis执行过的所有写指令记录下来(读操作不记录)，只许追加文件但不可以改写文件，redis启动之初会读取该文件重新构建数据，换言之，redis重启的话就根据日志文件的内容将写指令从前到后执行一次以完成数据的恢复工作。

二：Redis配置文件redis.conf中关于AOF相关配置

############################## APPEND ONLY MODE ###############################

# By default Redis asynchronously dumps the dataset on disk. This mode is
# good enough in many applications, but an issue with the Redis process or
# a power outage may result into a few minutes of writes lost (depending on
# the configured save points).
#
# The Append Only File is an alternative persistence mode that provides
# much better durability. For instance using the default data fsync policy
# (see later in the config file) Redis can lose just one second of writes in a
# dramatic event like a server power outage, or a single write if something
# wrong with the Redis process itself happens, but the operating system is
# still running correctly.
#
# AOF and RDB persistence can be enabled at the same time without problems.
# If the AOF is enabled on startup Redis will load the AOF, that is the file
# with the better durability guarantees.
#
# Please check http://redis.io/topics/persistence for more information.

appendonly no

# The name of the append only file (default: "appendonly.aof")

appendfilename "appendonly.aof"

# The fsync() call tells the Operating System to actually write data on disk
# instead of waiting for more data in the output buffer. Some OS will really flush
# data on disk, some other OS will just try to do it ASAP.
#
# Redis supports three different modes:
#
# no: don't fsync, just let the OS flush the data when it wants. Faster.
# always: fsync after every write to the append only log. Slow, Safest.
# everysec: fsync only one time every second. Compromise.
#
# The default is "everysec", as that's usually the right compromise between
# speed and data safety. It's up to you to understand if you can relax this to
# "no" that will let the operating system flush the output buffer when
# it wants, for better performances (but if you can live with the idea of
# some data loss consider the default persistence mode that's snapshotting),
# or on the contrary, use "always" that's very slow but a bit safer than
# everysec.
#
# More details please check the following article:
# http://antirez.com/post/redis-persistence-demystified.html
#
# If unsure, use "everysec".

# appendfsync always
appendfsync everysec
# appendfsync no

# When the AOF fsync policy is set to always or everysec, and a background
# saving process (a background save or AOF log background rewriting) is
# performing a lot of I/O against the disk, in some Linux configurations
# Redis may block too long on the fsync() call. Note that there is no fix for
# this currently, as even performing fsync in a different thread will block
# our synchronous write(2) call.
#
# In order to mitigate this problem it's possible to use the following option
# that will prevent fsync() from being called in the main process while a
# BGSAVE or BGREWRITEAOF is in progress.
#
# This means that while another child is saving, the durability of Redis is
# the same as "appendfsync none". In practical terms, this means that it is
# possible to lose up to 30 seconds of log in the worst scenario (with the
# default Linux settings).
#
# If you have latency problems turn this to "yes". Otherwise leave it as
# "no" that is the safest pick from the point of view of durability.

no-appendfsync-on-rewrite no

# Automatic rewrite of the append only file.
# Redis is able to automatically rewrite the log file implicitly calling
# BGREWRITEAOF when the AOF log size grows by the specified percentage.
#
# This is how it works: Redis remembers the size of the AOF file after the
# latest rewrite (if no rewrite has happened since the restart, the size of
# the AOF at startup is used).
#
# This base size is compared to the current size. If the current size is
# bigger than the specified percentage, the rewrite is triggered. Also
# you need to specify a minimal size for the AOF file to be rewritten, this
# is useful to avoid rewriting the AOF file even if the percentage increase
# is reached but it is still pretty small.
#
# Specify a percentage of zero in order to disable the automatic AOF
# rewrite feature.

auto-aof-rewrite-percentage 100
auto-aof-rewrite-min-size 64mb

# An AOF file may be found to be truncated at the end during the Redis
# startup process, when the AOF data gets loaded back into memory.
# This may happen when the system where Redis is running
# crashes, especially when an ext4 filesystem is mounted without the
# data=ordered option (however this can't happen when Redis itself
# crashes or aborts but the operating system still works correctly).
#
# Redis can either exit with an error when this happens, or load as much
# data as possible (the default now) and start if the AOF file is found
# to be truncated at the end. The following option controls this behavior.
#
# If aof-load-truncated is set to yes, a truncated AOF file is loaded and
# the Redis server starts emitting a log to inform the user of the event.
# Otherwise if the option is set to no, the server aborts with an error
# and refuses to start. When the option is set to no, the user requires
# to fix the AOF file using the "redis-check-aof" utility before to restart
# the server.
#
# Note that if the AOF file will be found to be corrupted in the middle
# the server will still exit with an error. This option only applies when
# Redis will try to read more data from the AOF file but not enough bytes
# will be found.
aof-load-truncated yes

这里对配置文件不懂的可以看这里：redis.conf 详解

1.默认AOF没有开启

	appendonly no #如果要开启，改为yes

• 启动：修改为yes，启动，这里要注意的是启动的目录和保存aof文件目录是否一致！查看目录命令(config get dir)，这一点在上一篇RDB中讲过，不懂的请看上一篇RDB中3:配置位置，以及快照恢复
• 修复：使用redis-check-aof --fix 进行修复
• 恢复：重启redis然后重新加载

2.默认名称

	appendonlyfilename   appendonly.aof

3.三种appendfsysnc：同步策略

	#always：同步持久化，每次发生数据变更会立即记录到磁盘，性能较差到数据完整性比较好
	everysec：出厂的默认推荐，异步同步，每秒记录一次
	#no：不同步

三：重写（rewrite）（查看Linux服务器命令-free内存， df磁盘），时间换空间的思想在里面！

1.重写是什么？

 AOF采用文件追加方式，文件会越来越大为避免出现此种情况，新增了重写机制,当AOF文件的大小超过所设定的阈值时，Redis就会启动AOF文件的内容压缩，只保留可以恢复数据的最小指令集.可以使用命令bgrewriteaof！

2.重写原理

	AOF文件持续增长而过大时，会fork出一条新进程来将文件重写(也是先写临时文件最后再rename)，
遍历新进程的内存中数据，每条记录有一条的Set语句。重写aof文件的操作，并没有读取旧的aof文件，
而是将整个内存中的数据库内容用命令的方式重写了一个新的aof文件，这点和快照有点类似

3.触发机制

Redis会记录上次重写时的AOF大小，默认配置是当AOF文件大小是上次rewrite后大小的一倍且文件大于64M时触发。默认64M。
--看触发机制配置，知道公司实力！

四：优点

• 每修改同步：appendfsync always 同步持久化 每次发生数据变更会被立即记录到磁盘 性能较差但数据完整性比较好
• 每秒同步：appendfsync everysec 异步操作，每秒记录 如果一秒内宕机，有数据丢失
• 不同步：appendfsync no 从不同步

五：缺点

• 相同数据集的数据而言aof文件要远大于rdb文件，恢复速度慢于rdb
• aof运行效率要慢于rdb,每秒同步策略效率较好，不同步效率和rdb相同

六：总结

客户端--->命令请求--->服务器 ------->网络协议格式的命令内容-->AOF文件

• AOF 文件是一个只进行追加的日志文件
• Redis可以在AOF文件体积变得过大时，自动地在后台对AOF进行重写
• AOF文件有序地保存了对数据库执行所有写入操作，这些写入操作作为redis协议的格式保存，因此AOF文件的内容非常容易被人读懂，对文件进行分析也很轻松
• 对于相同的数据集来说，AOF文件的体积通常大于RDB文件的体积，根据所使用的fsync策略，AOF的速度可能会慢于RDB

下面是彩蛋 、

最后进行一个总的总结那么在使用redis持久化的时候到底该怎么选择？

1：到底选择哪个？

官网：http://www.redis.cn/topics/persistence.html

• RDB持久化方式能够在指定的时间间隔能对你的数据进行快照存储
• AOF持久化方式记录每次对服务器写的操作,当服务器重启的时候会重新执行这些命令来恢复原始的数据,AOF命令以redis协议追加保存每次写的操作到文件末尾.Redis还能对AOF文件进行后台重写,使得AOF文件的体积不至于过大。

只做缓存：如果你只希望你的数据在服务器运行的时候存在,你也可以不使用任何持久化方式.

同时开启两种持久化方式

--在这种情况下,当redis重启的时候会优先载入AOF文件来恢复原始的数据,

因为在通常情况下AOF文件保存的数据集要比RDB文件保存的数据集要完整.

--RDB的数据不实时，同时使用两者时服务器重启也只会找AOF文件。那要不要只使用AOF呢？

作者建议不要，因为RDB更适合用于备份数据库(AOF在不断变化不好备份)，

快速重启，而且不会有AOF可能潜在的bug，留着作为一个万一的手段。

2.性能建议

• 因为RDB文件只用作后备用途，建议只在Slave上持久化RDB文件，而且只要15分钟备份一次就够了，只保留save 900 1这条规则。
• 如果Enalbe AOF，好处是在最恶劣情况下也只会丢失不超过两秒数据，启动脚本较简单只load自己的AOF文件就可以了。代价一是带来了持续的IO，二是AOF rewrite的最后将rewrite过程中产生的新数据写到新文件造成的阻塞几乎是不可避免的。只要硬盘许可，应该尽量减少AOF rewrite的频率，AOF重写的基础大小默认值64M太小了，可以设到5G以上。默认超过原大小100%大小时重写可以改到适当的数值。
• 如果不Enable AOF ，仅靠Master-Slave Replication 实现高可用性也可以。能省掉一大笔IO也减少了rewrite时带来的系统波动。代价是如果Master/Slave同时倒掉，会丢失十几分钟的数据，启动脚本也要比较两个Master/Slave中的RDB文件，载入较新的那个。新浪微博就选用了这种架构

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