Redis从入门到放弃系列(六) 持久化
本文例子基于：5.0.4 Redis持久化机制包括两种,RDB和AOF,当两种机制都存在的时候,redis启动时会采用aof来恢复数据
Redis为了保证数据的持久性,提供了RDB跟AOF机制,RDB是内存数据的二进制序列化形式,在存储上非常紧凑,AOF日志记录的是数据修改的指令记录文本。
RDB
RDB机制其具体实现为使用系统的多进程写时复制(Copy On Write)，写时复制是一种可以推迟甚至避免拷贝数据的技术。内核此时并不复制整个进程的地址空间，而是让父子进程共享同一个地址空间。只用在需要写入的时候才会复制地址空间，从而使各个进行拥有各自的地址空间。
我们知道redis是单线程的,当redis在接受请求的时候还要处理rdb save,如果数据量很大,那么对于业务的影响是致命的,所以redis采用了多进程写时复制技术来解决该问题.父进程首先fork一个子进程，RDB持久化交给了子进程来进行,父进程继续处理客户端的请求.
那么redis在什么时候会触发RDB持久化呢? 我们来看一下redis.conf里面关于RDB持久化的说明：
################################ 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 ./
我们可以看到，redis默认有3个措施触发RDB的持久化：
- 60秒有10000个key改变
- 300秒有10个key改变
- 900秒有1个key改变 当以上任意条件满足则触发RDB持久化机制.如果不需要RDB持久化,可以把该配置注释掉.RDB最严重的情况下会存在redis占据原先数据内存的2倍大小。 RDB默认启用LZF压缩字符串对象,以节省储存空间。
AOF
让我们先来了解一下redis.conf里面对于aof持久化的配置说明:
############################## 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
# 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.
aof-use-rdb-preamble yes
AOF日志记录了服务器接收到的每一次写入操作，这些操作会在redis服务器启动的时候重新执行一遍,重构原始数据集。通过只能追加的方式,使用与redis协议相同的格式记录。当aof日志文件过大的时候，会触发aof-rewriete。
触发aof持久化有三种策略:
- appendfsync always(当有写入操作就触发一次持久化)
- appendfsync everysec(每秒持久化一次,生产使用)
- appendfsync no(不进行同步,让操作系统决定在需要的时候将数据持久化)
当aof日志文件过大时会在后台自动重写它。aof-rewrite是安全的。首先redis会开启一个子进程对内存进行遍历转换成对应的redis的操作指令,将对应的操作指令序列化到一个新的aof日志文件中。当序列化完成之后,redis会将序列化期间产生的aof变化同步到该aof文件中，当同步完毕之后替代原本旧的aof文件.
触发aof-rewrite配置: - no-appendfsync-on-rewrite(重写时是否可以运行Appendfsync) - auto-aof-rewrite-percentage(当文件大小是上次rewrite后的大小的百分比) - auto-aof-rewrite-min-size(当aof文件不小于该数值)
在redis4.0之后启用了混合持久化. 前一部分是RDB格式,后一部分是AOF,即在持久化的时候,接受客户端请求过来的另外的缓存数据~
aof-use-rdb-preamble yes
