春招必问的redis持久化(RDB AOF)，你能答上来么？

　　春招面试模拟，如同雷同，纯属巧合！！！

　　面试的大体流程：

　　第一步：一般会有笔试题，也可能没有。有笔试题就要好好答了，因为会重视笔试结果，为了节约面试官时间，HR可能先会看，不合格直接让你走人了。

　　第二步：开始面试，面试官会让你先来个自我介绍，他在看你的简历。可能有人会有疑问？为什么简历上都写的很清楚还让我做自我介绍？因为面试官不会提前看你的简历，他要有时间去看简历。所以自我介绍，不宜过长或过短，要重点突出，怎么重点突出？提前看好岗位要求，要求都是入职之后用到的技术，所以面试官会看重那些！！！自我介绍结束，步入正题。

　　面试官会问：之前的公司用过redis么？

　　面试者有两类回答：

　　面试者甲：没用过，但自己学过，下载过源码，自己部署安装过，基本的命令像string/hash/lsit/set/zset,都熟悉；

　　面试官(心想虽然没用，但动手能力很强，也很好学，不错)：redis是内存数据库，那它怎么进行持久化的？

　　面试官甲（心想没注意看啊，不知道啊）：...

　　面试者乙：用过，比较熟悉

　　面试官会接着问：持久化方式有哪些？

　　面试者乙：RDB和AOF

　　面试官：原理？区别？优缺点说一下吧？

　　面试官乙：...

　　要是是你去面试，你能回答上来么？

　　下面让我为你一一揭晓答案！！！

　　RDB持久化(Redis DataBase)

　　redis是内存数据库，一旦服务器进程退出，服务器中的数据库状态也会消失不见。重点介绍save和bgsave命令。

　　RDB文件的创建与载入

　　save命令会阻塞Redis服务器进程，直到RDB文件创建为止，在服务器进程阻塞期间，服务器不能处理任何命令请求。

　　bgsave命令派生出子进程，，然后由子进程创建RDB文件，父进程继续处理请求。

　　创建RDB文件实际rdb.c/rdbLoad函数完成的

　　自动间隔性保存

　　redis服务器会通过用户配置save选项，每隔一段时间去执行一下bgsave命令；默认的配置文件redis.conf，关于rdb部分的配置如下：

################################ 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 ./

　　主要有三条策略，满足任意一个，就会执行bgave命令：

　　服务器在900秒之内，对数据库至少修改了1次

　　服务器在300秒之内，对数据库至少修改了10次

　　服务器在60秒之内，对数据库至少修改了10000次

　　优缺点

　　优点：

　　适合大规模的数据恢复

　　对数据完整性和一致性要求不高

　　缺点：

　　会丢失最后一次修改的数据

　　fork会产生额外消耗

　　AOF持久化（Append Only File）

　　与RDB通过键值对来记录数据库状态不同，AOF是通过Redis服务器所执行的写命令来记录数据库状态的。

　　AOF持久化的实现

　　AOF持久化功能的实现可以分为命令追加(append)、文件写入、文件同步(sync)三个步骤。

　　aof在redis.conf配置文件的：

　　

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

# 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

　　AOF重写

　　随着服务器时间的流逝，文件的体积越来越大，体积过大的AOF文件对redis服务器、甚至整个宿主计算机造成影响。并且AOF文件的体积越大，使用AOF文件进行数据还原所需的时间越多。

　　为了解决AOF文件体重膨胀的问题，redis提供了AOF文件重写(rewrite)的功能。

　　触发机制

　　redis会记录上次重写时AOF的大小，默认配置是当AOF文件大小是上次rewrite后大小的一倍且大于64M；默认配置如下：　　

　　auto-aof-rewrite-percentage 100
　　auto-aof-rewrite-min-size 64mb

　　悄悄告诉你们个小秘密：可能在面试时，面试官会吹嘘公司很牛，redis用的出神入化，当你入职之后，可以悄悄看看“auto-aof-rewrite-min-size 64mb”，默认64M大小，根本不够用，告诉发展的公司起码要3G起。

　　优缺点

　　优点：

　　配置灵活，可以选择多种方式进行持久化

　　缺点：　　

　　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