Redis持久化

Redis持久化

1. RDB

RDB全称Redis Database Backup file（Redis数据备份文件），也被叫做Redis数据快照，简单来说就是把内存中的所有数据都记录到磁盘中。当Redis故障重启后，此磁盘读取快照文件，恢复数据。

1.1 主动备份：

• save：Redis主进程来执行RDB，会阻塞所有命令
• bgsave：开启子进程来执行RDB，避免主进程受到影响。

127.0.0.1:6379> save
OK
127.0.0.1:6379> bgsave
Background saving started
127.0.0.1:6379>

1.2 触发备份：

redis.conf文件中，

################################ SNAPSHOTTING  ################################
#
# 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 ""
# 900秒内，如果至少1个key被修改，则执行bgsave
save 900 1
save 300 10
save 60 10000

1.3 RDB原理

bgsave开始时会fork主进程得到子进程，子进程共享主进程的内存数据。完成fork后读取内存数据写入RDB文件。

fork采用的是copy-on-write技术：

• 当主进程执行读操作时，访问共享内存（read-only）；
• 当主进程执行写操作时，则拷贝一份数据（数据副本），执行写操作；

页表：记录虚拟地址与物理地址的映射关系。

2. AOF

AOF全称为Append Only File（追加文件），Redis处理的每一个写命令都会记录在AOF文件，可以看做是命令日志文件。

AOF默认是关闭的，在redis.conf中开启

############################## 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.
#是否开启AOF功能，默认是no
appendonly no

# The name of the append only file (default: "appendonly.aof")
# AOF文件名称
appendfilename "appendonly.aof"

2.1 AOF命令的频率

# 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".

# 表示每执行一次写命令，立即记录到AOF文件
# appendfsync always

# 写命令执行完成先放入AOF缓冲区，然后表示每隔1秒将缓冲区数据写到AOF文件，是默认方案
appendfsync everysec

# 写命令执行完先放入AOF缓冲区，由操作系统决定何时将缓冲区内容写回磁盘
# appendfsync no

配置项	刷盘时机	优点	缺点
always	同步刷盘	可靠性高，几乎不丢数据	性能影响大
everysec（默认）	每秒刷盘	性能适中	最多丢失1秒数据
no	操作系统控制	性能最好	可靠性较差，可能丢失大量数据

2.2 bgrewriteaof命令

让AOF执行文件重写功能，用最少得命令达到相同的效果。
用来处理对一个key多次写操作

2.3 AOF触发阈值配置

redis.conf文件中

# 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文件比上次文件增长超过多少百分比则触发重写
auto-aof-rewrite-percentage 100

# AOF文件体积最小多大以上才出发重写
auto-aof-rewrite-min-size 64mb

3. RDB与AOF对比

RDB和AOF各有优缺点，如果对数据安全性要求较高，在时机开发中往往会结合两者来使用

	RDB	AOF
持久化方式	定时对整个内存做快照	记录每一次执行的命令
数据完整性	不完整，两次备份之间会丢失	相对完整，取决于刷盘策略
文件大小	会有压缩，文件体积小	记录命令，文件体积很大
宕机恢复速度	很快	慢
数据恢复优先级	低，因为数据完整性不如AOF	高，因为数据完整性更高
系统资源占用	高，大量cpu和内存消耗	低，主要是磁盘IO资源，但AOF重写时会占用大量cpu和内存资源
使用场景	可以容忍数分钟的数据丢失，追求更快启动速度	对数据安全性要求较高