Redis 数据的持久化 RDB、AOF、RDB + AOF、No persistence 各自优缺点

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持久化，是指将Redis内存中的数据写入磁盘，当遇到重启Redis或重启服务器时，再将磁盘上的数据恢复到内存中

Redis持久化选项有4种
RDB (Redis Database) 以指定的时间间隔进行数据的快照备份。
AOF (Append Only File) 记录更改数据的命令（例如SET）, 然后在重启时再次执行这些命令，从而恢复数据。
RDB + AOF 组合使用
No persistence 禁用持久化

一、RDB (Redis Database)

RDB开启后，Redis 可以自动创建某个时间点的快照，以便重启的时候，将快照中的数据恢复在内存中。

1.1 RDB 优势

• 以快照的形式备份，非常适合灾难后进行数据的恢复；大数据时，更快地重新启动。
• 因为是以时间节点的形式备份，节省了资源，也最大限度地提高了Redis的性能。

1.2 RDB 缺点

• 因为是以快照的形式备份，会阻塞线程，来保证数据完整性，当遇到大数据时，阻塞时间会被放大。
• 以时间节点的形式处理，服务停止工作时，可能还没到达时间点，会导致部分数据丢失，只能恢复到上一个时间节点上。
• Redis运行时，手动复制RDB文件是安全的，那怕Redis正在写入临时RDB文件，原始的RDB文件也是完整的。
• 每个时刻，RDB文件会有一份，可以创建cron任务，备份RDB文件。记录不同时刻的RDB文件。

1.3 RDB 如何工作

达到配置中save指令条件时，子进程将内存中的数据集写入临时RDB文件，再进行原子替换操作，替换旧的RDB文件。 此时RDB文件只会有一份。

1.4 RDB配置

以windows版为例， 在redis.conf中 （redis.windows-service.conf 和 redis.windows.conf）

redis.windows-service.conf 旨在作为服务/守护进程运行。这意味着它应该在后台运行并由操作系统管理(重启时启动，崩溃时重新启动等)。

redis.windows.conf 旨在从命令行或脚本运行并在用户空间中进行管理。

两个配置文件基本相同，更细的区别，不再赘述。

在windows服务管理中启用、停用redis服务，仅修改redis.windows-service.conf配置即可

################################ SNAPSHOTTING  ################################
#
# Save the DB on disk:
#
#   save <seconds> <changes>
#
#   Will save the DB if both the given number of seconds and the given
#   number of write operations against the DB occurred.
#
#   In the example below the behaviour will be to save:
#   after 900 sec (15 min) if at least 1 key changed
#   after 300 sec (5 min) if at least 10 keys changed
#   after 60 sec if at least 10000 keys changed
#
#   Note: you can disable saving completely by commenting out all "save" lines.
#
#   It is also possible to remove all the previously configured save
#   points by adding a save directive with a single empty string argument
#   like in the following example:
#
#   save ""save 900 1
save 300 10
save 60 10000# By default Redis will stop accepting writes if RDB snapshots are enabled
# (at least one save point) and the latest background save failed.
# This will make the user aware (in a hard way) that data is not persisting
# on disk properly, otherwise chances are that no one will notice and some
# disaster will happen.
#
# If the background saving process will start working again Redis will
# automatically allow writes again.
#
# However if you have setup your proper monitoring of the Redis server
# and persistence, you may want to disable this feature so that Redis will
# continue to work as usual even if there are problems with disk,
# permissions, and so forth.
stop-writes-on-bgsave-error yes# Compress string objects using LZF when dump .rdb databases?
# For default that's set to 'yes' as it's almost always a win.
# If you want to save some CPU in the saving child set it to 'no' but
# the dataset will likely be bigger if you have compressible values or keys.
rdbcompression yes# Since version 5 of RDB a CRC64 checksum is placed at the end of the file.
# This makes the format more resistant to corruption but there is a performance
# hit to pay (around 10%) when saving and loading RDB files, so you can disable it
# for maximum performances.
#
# RDB files created with checksum disabled have a checksum of zero that will
# tell the loading code to skip the check.
rdbchecksum yes# The filename where to dump the DB
dbfilename dump.rdb# The working directory.
#
# The DB will be written inside this directory, with the filename specified
# above using the 'dbfilename' configuration directive.
#
# The Append Only File will also be created inside this directory.
#
# Note that you must specify a directory here, not a file name.
dir ./

1.5 开启/关闭，RDB快照策略，save指令

# 900秒（15分钟）后，如果至少有一个按键发生变化
save 900 1  
# 300秒（5分钟）后，如果至少有10个按键发生变化  
save 300 10 
# 60秒后，如果至少有10000个密钥发生更改   
save 60 10000  

• 注释或删除掉所有策略，就可以禁用RDB模式
• 添加自定义策略，只需要满足save指令规则即可

1.6 持久化硬盘文件，dbfilename指令

dbfilename dump.rdb

指定了文件的名称、扩展名；dump.rdb是一个的二进制文件

1.7 持久化硬盘文件的存储地址，dir指令

dir ./

默认是：当前安装目录下

AOF和RDB模式，使用了这个公共配置项，如果修改RDB和AOF都受影响

二、AOF (Append Only File)

2.1 AOF 优势

• 持久化的同步有不同的策略，fsync策略，参照后面文章，可以自行灵活配置
• 即使由于某种原因（磁盘已满或其他原因），AOF文件中存在不完整命令，redis check aof工具也能很容易地修复它。
• AOF文件过大时，文件会被拆分和重写，用当前数据集所需的最小操作命令集生成一个全新的AOF文件，再进行原子替换操作，切换新旧AOF文件

2.2 AOF 缺点

• AOF文件通常比RDB文件大；因为RDB是时间点的数据快照，AOF是整个数据变动过程的完整命令。
• AOF重写过程时, 可能会占用更多内存, 先缓冲在内存中，并在最后写入新的AOF

2.3 AOF 如何工作

Redis 收到更改数据集的命令(例如SET)时，将该命令写入到缓冲区，最后依据fsync策略，从缓冲区再追加AOF文件中。需要恢复时，执行中AOF文件中命令即可。

Redis 7.0.0以后，AOF文件过大时，原始的单个AOF文件被拆分为基本文件（最多一个）和增量文件（可能有多个）

2.4 AOF配置

以windows版为例， 在redis.conf中 （redis.windows-service.conf 和 redis.windows.conf）

redis.windows-service.conf 旨在作为服务/守护进程运行。这意味着它应该在后台运行并由操作系统管理(重启时启动，崩溃时重新启动等)。

redis.windows.conf 旨在从命令行或脚本运行并在用户空间中进行管理。

两个配置文件基本相同，更细的区别，不再赘述。

在windows服务管理中启用、停用redis服务，仅修改redis.windows-service.conf配置即可

############################## 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

2.5 开启/关闭，appendonly指令

默认情况下 Redis 没有开启 AOF（Redis 6.0 之后已经默认是开启了）
通过 appendonly 参数开启：yes/no 开启/关闭

appendonly yes

2.6 AOF文件，appendfilename指令

appendfilename "appendonly.aof"

2.7 AOF文件目录，dir指令

dir ./

默认是：当前安装目录下

AOF和RDB模式，使用了这个公共配置项，如果修改RDB和AOF都受影响

2.8 AOF追加策略，appendfsync指令

appendfsync everysec  
# appendfsync always    
# appendfsync no        

• everysec 默认值, 后台线程每秒主动同步一次，将缓冲区数据写入到aof文件；性能和数据一致性的折中方案。

• always 每次缓冲区有更改数据集的命令时，直接写入AOF文件；性能差一点，但数据一致性更高。

• no 不主动进行写入到aof文件，等待操作系统刷新数据；数据一致比较差，性能更好；Linux 下一般为 30 秒一次。

2.9 AOF写入被意外中断，数据也能恢复到上一句命令，可靠性高

当写入AOF文件时，redis或服务器发生故障，可能导致命令写入了一半（命令不完整），当服务恢复时，redis能自己检测出来异常的命令，将其截断，然后将数据恢复到上一个命令记录的时刻，可能最多损失1秒的数据（配置是 appendfsync everysec）

* Reading RDB preamble from AOF file...
* Reading the remaining AOF tail...
# !!! Warning: short read while loading the AOF file !!!
# !!! Truncating the AOF at offset 439 !!!
# AOF loaded anyway because aof-load-truncated is enabled

三、No persistence 禁用持久化

在上面文章中，已经讲述到了RDB、AOF的开启关闭， 将RDB、AOF都关闭，其实就是不使用持久化。

关闭后，redis对于数据的操作，不会被同步到RDB、AOF文件；
关闭后，重启服务后，也不会从RDB、AOF文件恢复数据，所以重启服务后，redis没有数据。
关闭后，向redis写入key-val数据，如果重启服务，redis就回丢失数据

如果再次开启RDB、AOF模式，重启服务后，会再次加载RDB、AOF文件中的数据。

四、RDB 和 AOF 能否同时开启？

AOF和RDB可以同时启用

AOF文件将优先使用，因为它保证是最完整的

五、如何选择 RDB 和 AOF？

个人理解，RDB是必须要开启的，Redis默认也是开启的， 简单的快照是一个有效的恢复手段，哪怕中间有小部分数据丢失。

普通场景, 能承受一定的数据丢失，可能只开启RDB即可。
数据安全性要求高的场景，可能需要RDB和AOF同时开启，

任何时候，都应该再借助数据库（mqsql、 sql server）进行持久化，做最后一道防线。

六、官方文档

https://redis.io/docs/management/persistence/

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