春招必问的redis持久化(RDB AOF),你能答上来么?

2019/04/10 10:10
阅读数 27

  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:

 1 ################################ SNAPSHOTTING  ################################
 2 #
 3 # Save the DB on disk:
 4 #
 5 #   save <seconds> <changes>
 6 #
 7 #   Will save the DB if both the given number of seconds and the given
 8 #   number of write operations against the DB occurred.
 9 #
10 #   In the example below the behaviour will be to save:
11 #   after 900 sec (15 min) if at least 1 key changed
12 #   after 300 sec (5 min) if at least 10 keys changed
13 #   after 60 sec if at least 10000 keys changed
14 #
15 #   Note: you can disable saving completely by commenting out all "save" lines.
16 #
17 #   It is also possible to remove all the previously configured save
18 #   points by adding a save directive with a single empty string argument
19 #   like in the following example:
20 #
21 #   save ""
22 
23 save 900 1
24 save 300 10
25 save 60 10000
26 
27 # By default Redis will stop accepting writes if RDB snapshots are enabled
28 # (at least one save point) and the latest background save failed.
29 # This will make the user aware (in a hard way) that data is not persisting
30 # on disk properly, otherwise chances are that no one will notice and some
31 # disaster will happen.
32 #
33 # If the background saving process will start working again Redis will
34 # automatically allow writes again.
35 #
36 # However if you have setup your proper monitoring of the Redis server
37 # and persistence, you may want to disable this feature so that Redis will
38 # continue to work as usual even if there are problems with disk,
39 # permissions, and so forth.
40 stop-writes-on-bgsave-error yes
41 
42 # Compress string objects using LZF when dump .rdb databases?
43 # For default that's set to 'yes' as it's almost always a win.
44 # If you want to save some CPU in the saving child set it to 'no' but
45 # the dataset will likely be bigger if you have compressible values or keys.
46 rdbcompression yes
47 
48 # Since version 5 of RDB a CRC64 checksum is placed at the end of the file.
49 # This makes the format more resistant to corruption but there is a performance
50 # hit to pay (around 10%) when saving and loading RDB files, so you can disable it
51 # for maximum performances.
52 #
53 # RDB files created with checksum disabled have a checksum of zero that will
54 # tell the loading code to skip the check.
55 rdbchecksum yes
56 
57 # The filename where to dump the DB
58 dbfilename dump.rdb
59 
60 # The working directory.
61 #
62 # The DB will be written inside this directory, with the filename specified
63 # above using the 'dbfilename' configuration directive.
64 #
65 # The Append Only File will also be created inside this directory.
66 #
67 # Note that you must specify a directory here, not a file name.
68 dir ./
rdb

  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:

  

  1 ############################## APPEND ONLY MODE ###############################
  2 
  3 # By default Redis asynchronously dumps the dataset on disk. This mode is
  4 # good enough in many applications, but an issue with the Redis process or
  5 # a power outage may result into a few minutes of writes lost (depending on
  6 # the configured save points).
  7 #
  8 # The Append Only File is an alternative persistence mode that provides
  9 # much better durability. For instance using the default data fsync policy
 10 # (see later in the config file) Redis can lose just one second of writes in a
 11 # dramatic event like a server power outage, or a single write if something
 12 # wrong with the Redis process itself happens, but the operating system is
 13 # still running correctly.
 14 #
 15 # AOF and RDB persistence can be enabled at the same time without problems.
 16 # If the AOF is enabled on startup Redis will load the AOF, that is the file
 17 # with the better durability guarantees.
 18 #
 19 # Please check http://redis.io/topics/persistence for more information.
 20 
 21 appendonly no
 22 
 23 # The name of the append only file (default: "appendonly.aof")
 24 
 25 appendfilename "appendonly.aof"
 26 
 27 # The fsync() call tells the Operating System to actually write data on disk
 28 # instead of waiting for more data in the output buffer. Some OS will really flush
 29 # data on disk, some other OS will just try to do it ASAP.
 30 #
 31 # Redis supports three different modes:
 32 #
 33 # no: don't fsync, just let the OS flush the data when it wants. Faster.
 34 # always: fsync after every write to the append only log. Slow, Safest.
 35 # everysec: fsync only one time every second. Compromise.
 36 #
 37 # The default is "everysec", as that's usually the right compromise between
 38 # speed and data safety. It's up to you to understand if you can relax this to
 39 # "no" that will let the operating system flush the output buffer when
 40 # it wants, for better performances (but if you can live with the idea of
 41 # some data loss consider the default persistence mode that's snapshotting),
 42 # or on the contrary, use "always" that's very slow but a bit safer than
 43 # everysec.
 44 
 45 # More details please check the following article:
 46 # http://antirez.com/post/redis-persistence-demystified.html
 47 #
 48 # If unsure, use "everysec".
 49 
 50 # appendfsync always
 51 appendfsync everysec
 52 # appendfsync no
 53 
 54 # When the AOF fsync policy is set to always or everysec, and a background
 55 # saving process (a background save or AOF log background rewriting) is
 56 # performing a lot of I/O against the disk, in some Linux configurations
 57 # Redis may block too long on the fsync() call. Note that there is no fix for
 58 # this currently, as even performing fsync in a different thread will block
 59 # our synchronous write(2) call.
 60 #
 61 # In order to mitigate this problem it's possible to use the following option
 62 # that will prevent fsync() from being called in the main process while a
 63 # BGSAVE or BGREWRITEAOF is in progress.
 64 #
 65 # This means that while another child is saving, the durability of Redis is
 66 # the same as "appendfsync none". In practical terms, this means that it is
 67 # possible to lose up to 30 seconds of log in the worst scenario (with the
 68 # default Linux settings).
 69 #
 70 # If you have latency problems turn this to "yes". Otherwise leave it as
 71 # "no" that is the safest pick from the point of view of durability.
 72 
 73 no-appendfsync-on-rewrite no
 74 
 75 # Automatic rewrite of the append only file.
 76 # Redis is able to automatically rewrite the log file implicitly calling
 77 # BGREWRITEAOF when the AOF log size grows by the specified percentage.
 78 #
 79 # This is how it works: Redis remembers the size of the AOF file after the
 80 # latest rewrite (if no rewrite has happened since the restart, the size of
 81 # the AOF at startup is used).
 82 #
 83 # This base size is compared to the current size. If the current size is
 84 # bigger than the specified percentage, the rewrite is triggered. Also
 85 # you need to specify a minimal size for the AOF file to be rewritten, this
 86 # is useful to avoid rewriting the AOF file even if the percentage increase
 87 
 88 # Specify a percentage of zero in order to disable the automatic AOF
 89 # rewrite feature.
 90 
 91 auto-aof-rewrite-percentage 100
 92 auto-aof-rewrite-min-size 64mb
 93 
 94 # An AOF file may be found to be truncated at the end during the Redis
 95 # startup process, when the AOF data gets loaded back into memory.
 96 # This may happen when the system where Redis is running
 97 # crashes, especially when an ext4 filesystem is mounted without the
 98 # data=ordered option (however this can't happen when Redis itself
 99 # crashes or aborts but the operating system still works correctly).
100 #
101 # Redis can either exit with an error when this happens, or load as much
102 # data as possible (the default now) and start if the AOF file is found
103 # to be truncated at the end. The following option controls this behavior.
104 #
105 # If aof-load-truncated is set to yes, a truncated AOF file is loaded and
106 # the Redis server starts emitting a log to inform the user of the event.
107 # Otherwise if the option is set to no, the server aborts with an error
108 # and refuses to start. When the option is set to no, the user requires
109 # to fix the AOF file using the "redis-check-aof" utility before to restart
110 # the server.
111 #
112 # Note that if the AOF file will be found to be corrupted in the middle
113 # the server will still exit with an error. This option only applies when
114 # Redis will try to read more data from the AOF file but not enough bytes
115 # will be found.
116 aof-load-truncated yes
aof

  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

  

  

原文出处:https://www.cnblogs.com/liudw-0215/p/12321540.html

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