redis持久化之RDB与AOF

aof，rdb是两种 redis持久化的机制。对内存数据库而言,系统宕机、硬件损坏等都会对数据造成损失。而redis对此有很强的修复能力,RDB （Redis DataBase）和 AOF （Append Only File）就是针对此类问题的解决方案。

RDB

Redis DataBase：在指定的时间间隔内将内存中的数据集快照写入磁盘，也就是行话讲的Snapshot快照，它恢复时是将快照文件直接读到内存里.

Redis会单独创建（Fork）一个子进程来进行持久化，会先将数据写入到一个临时文件中，待持久化过程都结束了，再用这个临时文件替换上次持久化好的文件。整个过程中，主进程是不进行任何IO操作的，这就确保了极高的性能
如果需要进行大规模数据的恢复，且对于数据恢复的完整性不是非常敏感，那RDB方式要比AOF方式更加的高效。RDB的缺点是最后一次持久化后的数据可能丢失。

Fork的作用是复制一个与当前进程一样的进程。新进程的所有数据（变量、环境变量、程序计数器等）数值都和原进程一致，但是是一个全新的进程，并作为原进程的子进程

配置文件中所处的位置：

################################ 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 秒钟 操作次数
save 900 1
save 300 10
save 60 10000

首先RDB 保存的是dump.rdb文件,RDB是整个内存压缩过的Snapshot,RDB的数据结构,可以配置复合的快照触发条件,出厂默认是一分钟内改一万次,或五分钟内改十次，或十五分钟内改一次。

Redis的Save命令可以快速备份,也就是说不用等到配置文件中配置的时间点,手动操作备份数据.FlushAll命令也会触发dump.rdb文件的改写(会清空掉)。Shutdown后也会生成dump.rdb文件.备份的文件所在备机和要恢复的主机最好不是同一台服务器.

Save：save时只管保存，其它不管，全部阻塞（在保存的这个时间范围内,Redis只管保存,不处理写入）
BGSAVE：Redis会在后台异步进行快照操作，快照同时还可以响应客户端请求。可以通过lastsave命令获取最后一次成功执行快照的时间
执行flushall命令，也会产生dump.rdb文件，但里面是空的，无意义

redis-server redis.windows.conf //redis启动后,会自动加载dump.rdb文件实现数据的恢复

AOF

概念：以日志的形式来记录每个写操作，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重启后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

配置文件所处位置：

############################## 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 //默认不使用

# The name of the append only file (default: "appendonly.aof") //默认的文件名字为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  //每一次写的操作就立即同步
appendfsync everysec  //默认每秒同步数据
# appendfsync no  //不同步

# 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.

#重写机制,以下两个是触发条件
auto-aof-rewrite-percentage 100  //上次重写后的一倍
auto-aof-rewrite-min-size 64mb  //文件大于64M

# 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

dump.rdb与appendononly.aof可以共存,先加载的是appendonly.aof文件

文件修复：Redis-check-aof --fix appendonly.aof //当出现文件错损时就可以执行修复,redis会修复不规范的语法
        redis-check-dump是用来修复dump.rdb文件的

aof的重写机制

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(redis会将重复的写的指令进行去重等措施来重写aof文件).
也可以使用命令bgrewriteaof来重写。
AOF进行重写是首先会有两个触发机制：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,默认是这样,但我们也可以改配置文件。

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

aof与rdb比较：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

 如有不当,望包涵！

😉