春招面試模擬，如同雷同，純屬巧合！！！

　　面試的大體流程：

　　第一步：一般會有筆試題，也可能沒有。有筆試題就要好好答了，因為會重視筆試結果，為了節約面試官時間，HR可能先會看，不合格直接讓你走人了。

　　第二步：開始面試，面試官會讓你先來個自我介紹，他在看你的簡歷。可能有人會有疑問？為什麼簡歷上都寫的很清楚還讓我做自我介紹？因為面試官不會提前看你的簡歷，他要有時間去看簡歷。所以自我介紹，不宜過長或過短，要重點突出，怎麼重點突出？提前看好崗位要求，要求都是入職之後用到的技術，所以面試官會看重那些！！！自我介紹結束，步入正題。

　　面試官會問：之前的公司用過redis麼？

　　面試者有兩類回答：

　　面試者甲：沒用過，但自己學過，下載過原始碼，自己部署安裝過，基本的命令像string/hash/lsit/set/zset,都熟悉；

　　面試官(心想雖然沒用，但動手能力很強，也很好學，不錯)：redis是記憶體資料庫，那它怎麼進行持久化的？

　　面試官甲（心想沒注意看啊，不知道啊）：...

　　面試者乙：用過，比較熟悉

　　面試官會接著問：持久化方式有哪些？

　　面試者乙：RDB和AOF

　　面試官：原理？區別？優缺點說一下吧？

　　面試官乙：...

　　要是是你去面試，你能回答上來麼？

　　下面讓我為你一一揭曉答案！！！

　　RDB持久化(Redis DataBase)

　　redis是記憶體資料庫，一旦伺服器程序退出，伺服器中的資料庫狀態也會消失不見。重點介紹save和bgsave命令。

　　RDB檔案的建立與載入

　　save命令會阻塞Redis伺服器程序，直到RDB檔案建立為止，在伺服器程序阻塞期間，伺服器不能處理任何命令請求。

　　bgsave命令派生出子程序，，然後由子程序建立RDB檔案，父程序繼續處理請求。

　　建立RDB檔案實際rdb.c/rdbLoad函式完成的

　　自動間隔性儲存

　　redis伺服器會通過使用者配置save選項，每隔一段時間去執行一下bgsave命令；預設的配置檔案redis.conf，關於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 ./

　　主要有三條策略，滿足任意一個，就會執行bgave命令：

　　伺服器在900秒之內，對資料庫至少修改了1次

　　伺服器在300秒之內，對資料庫至少修改了10次

　　伺服器在60秒之內，對資料庫至少修改了10000次

　　優缺點

　　優點：

　　適合大規模的資料恢復

　　對資料完整性和一致性要求不高

　　缺點：

　　會丟失最後一次修改的資料

　　fork會產生額外消耗

　　AOF持久化（Append Only File）

　　與RDB通過鍵值對來記錄資料庫狀態不同，AOF是通過Redis伺服器所執行的寫命令來記錄資料庫狀態的。

　　AOF持久化的實現

　　AOF持久化功能的實現可以分為命令追加(append)、檔案寫入、檔案同步(sync)三個步驟。

　　aof在redis.conf配置檔案的：

  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重寫

　　隨著伺服器時間的流逝，檔案的體積越來越大，體積過大的AOF檔案對redis伺服器、甚至整個宿主計算機造成影響。並且AOF檔案的體積越大，使用AOF檔案進行資料還原所需的時間越多。

　　為了解決AOF檔案體重膨脹的問題，redis提供了AOF檔案重寫(rewrite)的功能。

　　觸發機制

　　redis會記錄上次重寫時AOF的大小，預設配置是當AOF檔案大小是上次rewrite後大小的一倍且大於64M；預設配置如下：　　

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

　　悄悄告訴你們個小祕密：可能在面試時，面試官會吹噓公司很牛，redis用的出神入化，當你入職之後，可以悄悄看看“auto-aof-rewrite-min-size 64mb”，預設64M大小，根本不夠用，告訴發展的公司起碼要3G起。

　　優缺點

　　優點：

　　配置靈活，可以選擇多種方式進行持久化

　　缺點：　　

　　相同資料集的資料而言，aof檔案要遠大於rdb檔案，恢復速度慢於rdb