redis持久化机制（rdb 与 aof)

RDB Redis DataBase
在指定的时间间隔内将内存中的数据集快照到磁盘，生成dump.rdb文件，实际操作是一个fork子进程，先将数据集写入临时文件，写入成功后，再替换之前的文件，用二进制压缩
触发方式
手动触发

• save命令 ，使redis处于阻塞状态，直到rdb持久化完成，才响应其他客户端发来的命令，在生产环境要慎用
• bgsave命令，fork出一个子进程进行序列化，主进程只在fork过程中有短暂的阻塞，子进程创建后，主进程就可以响应客户端请i求了

自动触发

• save m n 在m秒内如果有n个键发生改变，则自动触发持久化，如果设置多个配置，只要满足其中一项就会触发
• flushall:用于清空redis数据库中的所有数据，flushdb清空当前redis所在的库数据，会清空rdb文件，同时也会生成一个dump.rdb文件内容为空。

优点

• 整个redis数据库只包含一个dump.rdb，方便持久化
• 容灾性好，方便备份
• 性能最大化，fork子进程来完成写操作，让主进程继续处理命令，使用单独的子进程进行持久化，主进程不会进行io操作，保证了redis的性能。
• 当数据集大时比aof的启动效率高

缺点

• 数据安全性低，RDB是以一种间隔的时间进行持久化，如果在时间间隔内数据库发生故障会发生数据丢失
• 因为是以fork子进程来进行持久化，当发生rdb时会禁止写操作，当数据集较大时，可能会导致整个服务器停止服务的时间变长。

我们可以在redis的配置文件redis.conf中发现rdb的配置及详细说明

################################ SNAPSHOTTING  ################################

rdb工作模式

# Save the DB on disk:
#
#   save  
#
#   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

rdb处理机制
# 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

rdb压缩方法及性能优化
# 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 "/www/server/redis"

AOF append of file 以日志的形式记录服务器所处理的每一个写，删除操作，记住命令，以文本的方式记录，可以打开看到详细的操作记录

• 所有的写命令会追加到aof缓冲中
• AOF缓冲区根据对应的策略向磁盘进行同步操作
• 随着AOF文件越来越大 需要定时对AOF进行重写，达到压缩的目的
• 当redis重启，可以调用aof文件进行数据恢复

同步策略

• 每秒同步 ：异步完成，效率非常高，一旦系统出现宕机现象，会损失这一秒内的数据
• 每次修改都同步：同步持久化，每次发生的数据都会被立即记录到磁盘中，最多损失最后一条数据
• 不同步 ：由操作系统控制，可能会丢失较多的数据

优点

• 数据安全
• 通过追加的方式写文件不会破坏已存在的内容，可以通过redis-check-aof解决一致性问题
• aof机制的rewrite模式，定期对aof文件进行重写，达到压缩的目的

缺点

• AOF文件比RDB文件大，恢复速度慢
• 数据集较大的时候，启动效率低
• 运行效率没有rdb高

Aof文件比RDB更新频率高，优先使用AOF进行数据还原
AOF比RDB更安全也更大
RDB性能比AOF好

aof在配置文件中的说明

############################## APPEND ONLY MODE ###############################
aof的一些优点和缺点
# 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.
默认不开启aof
appendonly no

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

appendfilename "appendonly.aof"

aof的持久化策略
# 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.
aof重写配置 当目前的aof文件时上此的百分之多少时进行重写
auto-aof-rewrite-percentage 100
重写文件的大小
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.
主机宕机后的操作 自动发布load文件
aof-load-truncated yes

# When rewriting the AOF file, Redis is able to use an RDB preamble in the
# AOF file for faster rewrites and recoveries. When this option is turned
# on the rewritten AOF file is composed of two different stanzas:
#
#   [RDB file][AOF tail]
#
# When loading Redis recognizes that the AOF file starts with the "REDIS"
# string and loads the prefixed RDB file, and continues loading the AOF
# tail.
加载时识别aof文件
aof-use-rdb-preamble yes