源码级别理解 Redis 持久化

{"type":"doc","content":[{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#595959","name":"user"}},{"type":"strong","attrs":{}}],"text":"❝ ","attrs":{}},{"type":"text","marks":[{"type":"color","attrs":{"color":"#595959","name":"user"}}],"text":"文章首发于公众号“蘑菇睡不着”，欢迎来访~ ❞","attrs":{}}]}],"attrs":{}},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#40B8FA","name":"user"}}],"text":"前言","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#2b2b2b","name":"user"}}],"text":"  大家都知道 Redis 是一个内存数据库，数据都存储在内存中，这也是 Redis 非常快的原因之一。虽然速度提上来了，但是如果数据一直放在内存中，是非常容易丢失的。比如 服务器关闭或宕机了，内存中的数据就木有了。为了解决这一问题，Redis 提供了 持久化 机制。分别是 ","attrs":{}},{"type":"text","marks":[{"type":"color","attrs":{"color":"#3594F7","name":"user"}},{"type":"strong","attrs":{}}],"text":"「RDB」","attrs":{}},{"type":"text","text":" 以及 ","attrs":{}},{"type":"text","marks":[{"type":"color","attrs":{"color":"#3594F7","name":"user"}},{"type":"strong","attrs":{}}],"text":"「AOF」","attrs":{}},{"type":"text","text":" 持久化。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#40B8FA","name":"user"}}],"text":"RDB","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#40B8FA","name":"user"}}],"text":"什么是 RDB 持久化?","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#2b2b2b","name":"user"}}],"text":"RDB 持久化可以在指定的时间间隔内生成数据集的时间点快照(point-in-time snapshot)。","attrs":{}}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/0f/0f8ee88669ba784ef0293673d0aa80f4.png","alt":"","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#40B8FA","name":"user"}}],"text":"RDB 的优点？","attrs":{}}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#595959","name":"user"}}],"text":"RDB 是一种表示某个即时点的 Redis 数据的紧凑文件。RDB 文件适用于备份。例如，你可能想要每小时归档最近24小时的 RDB 文件，每天保存近30天的 RDB 快照。这允许你很容易的恢复不同版本的数据集以容灾。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#595959","name":"user"}}],"text":"RDB 非常适合于灾难恢复，作为一个紧凑的单一文件，可以被传输到远程的数据中心。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#595959","name":"user"}}],"text":"RDB 最大化了 Redis 的性能。因为 Redis 父进程持久化时唯一需要做的是启动(fork)一个子进程，由子进程完成所有剩余的工作。父进程实例不需要执行像磁盘IO这样的操作。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#595959","name":"user"}}],"text":"RDB 在重启保存了大数据集的实例比 AOF 快。","attrs":{}}]}]}],"attrs":{}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#40B8FA","name":"user"}}],"text":"RDB 的缺点？","attrs":{}}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#595959","name":"user"}}],"text":"当你需要在Redis停止工作(例如停电)时最小化数据丢失，RDB可能不太好。你可以配置不同的保存点(save point)来保存RDB文件(例如，至少5分钟和对数据集100次写之后，但是你可以有多个保存点)。然而，你通常每隔5分钟或更久创建一个RDB快照，所以一旦Redis因为任何原因没有正确关闭而停止工作，你就得做好最近几分钟数据丢失的准备了。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#595959","name":"user"}}],"text":"RDB需要经常调用fork()子进程来持久化到磁盘。如果数据集很大的话，fork()比较耗时，结果就是，当数据集非常大并且CPU性能不够强大的话，Redis会停止服务客户端几毫秒甚至一秒。AOF也需要fork()，但是你可以调整多久频率重写日志而不会有损(trade-off)持久性(durability)。","attrs":{}}]}]}],"attrs":{}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#40B8FA","name":"user"}}],"text":"RDB 文件的创建与载入","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#2b2b2b","name":"user"}}],"text":"  有个两个 Redis 命令可以用于生成 RDB 文件，一个是 ","attrs":{}},{"type":"text","marks":[{"type":"color","attrs":{"color":"#3594F7","name":"user"}},{"type":"strong","attrs":{}}],"text":"「SAVE」","attrs":{}},{"type":"text","text":"，另一个是 ","attrs":{}},{"type":"text","marks":[{"type":"color","attrs":{"color":"#3594F7","name":"user"}},{"type":"strong","attrs":{}}],"text":"「BGSAVE」","attrs":{}},{"type":"text","text":"。  ","attrs":{}},{"type":"text","marks":[{"type":"color","attrs":{"color":"#3594F7","name":"user"}},{"type":"strong","attrs":{}}],"text":"「SAVE」","attrs":{}},{"type":"text","text":" 命令会阻塞 Redis 服务器进程，直到 RDB 文件创建完毕为止，在服务器进程阻塞期间，服务器不能处理任何命令请求。","attrs":{}}]},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"> SAVE     // 一直等到 RDB 文件创建完毕\nOK\n","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#2b2b2b","name":"user"}}],"text":"和 SAVE 命令直接阻塞服务器进程不同的是，BGSAVE 命令会派生出一个子进程，然后由子进程负责创建 RDB 文件，服务器进程(父进程)继续处理命令进程。","attrs":{}},{"type":"text","marks":[{"type":"color","attrs":{"color":"#3594F7","name":"user"}},{"type":"strong","attrs":{}}],"text":"「执行fork的时候操作系统（类Unix操作系统）会使用写时复制（copy-on-write）策略，即fork函数发生的一刻父子进程共享同一内存数据，当父进程要更改其中某片数据时（如执行一个写命令 ），操作系统会将该片数据复制一份以保证子进程的数据不受影响，所以新的RDB文件存储的是执行fork一刻的内存数据。」","attrs":{}}]},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"> BGSAVE  // 派生子进程，并由子进程创建 RDB 文件\nBackground saving started\n","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#2b2b2b","name":"user"}}],"text":"生成 RDB 文件由两种方式：","attrs":{}},{"type":"text","marks":[{"type":"color","attrs":{"color":"#3594F7","name":"user"}},{"type":"strong","attrs":{}}],"text":"「一种是手动，就是上边介绍的用命令的方式；另一种是自动的方式。」","attrs":{}},{"type":"text","text":"接下来详细介绍一下自动生成 RDB 文件的流程。Redis 允许用户通过设置服务器配置的 ","attrs":{}},{"type":"text","marks":[{"type":"color","attrs":{"color":"#3594F7","name":"user"}},{"type":"strong","attrs":{}}],"text":"「save」","attrs":{}},{"type":"text","text":" 选项，让服务器每隔一段时间自动执行一次 BGSAVE 命令。用户可以通过在 redis.conf 配置文件中的 SNAPSHOTTING 下 save 选项设置多个保存条件，但只要其中任意一个条件被满足，服务器就会执行 BGSAEVE 命令。如，以下配置：save 900 1save 300 10save 60 10000上边三个配置的含义是：","attrs":{}}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#595959","name":"user"}}],"text":"服务器在 900 秒内，对数据库进行了至少 1 次修改。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#595959","name":"user"}}],"text":"服务器在 300 秒内，对数据库进行了至少 10 次修改。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#595959","name":"user"}}],"text":"服务器在 60 秒内，对数据库进行了至少 10000 次修改。","attrs":{}}]}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#2b2b2b","name":"user"}}],"text":"如果没有手动去配置 save 选项，那么服务器会为 save 选项配置默认参数：save 900 1save 300 10save 60 10000接着，服务器就会根据 save 选项的配置，去设置服务器状态 redisServer 结构的 saveparams 属性：","attrs":{}}]},{"type":"codeblock","attrs":{"lang":"cpp"},"content":[{"type":"text","text":"struct redisServer{\n\n  // ...\n  \n  // 记录了保存条件的数组\n  struct saveparams *saveparams;\n  \n  // ...\n};\n","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#2b2b2b","name":"user"}}],"text":"saveparams 属性是一个数组，数组中的每一个元素都是一个 saveparam 结构，每个 saveparam 结构都保存了一个 save 选项设置的保存条件：","attrs":{}}]},{"type":"codeblock","attrs":{"lang":"cpp"},"content":[{"type":"text","text":"struct saveparam {\n\n  // 秒数\n  time_t seconds;\n  \n  // 修改数\n  int changes;\n};\n","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#2b2b2b","name":"user"}}],"text":"除了 saveparams 数组之外，服务器状态还维持着一个 dirty 计数器，以及一个 lastsave 属性；","attrs":{}}]},{"type":"codeblock","attrs":{"lang":"cpp"},"content":[{"type":"text","text":"struct redisServer {\n    // ...\n    \n    // 修改计数器\n    long long dirty;\n    \n    // 上一次执行保存时间\n     time_t lastsave;\n     \n     // ...\n}\n","attrs":{}}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#595959","name":"user"}}],"text":"dirty 计数器记录距离上一次成功执行 SAVE 或 BGSAVE 命令之后，服务器对数据库状态(服务器中的所有数据库)进行了多少次修改(包括写入、删除、更新等操作)。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#595959","name":"user"}}],"text":"lastsave 属性是一个 UNIX 时间戳，记录了服务器上一次执行 SAVE 或 BGSAVE 命令的时间。","attrs":{}}]}]}],"attrs":{}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#2b2b2b","name":"user"}}],"text":"检查条件是否满足触发 RDB","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#2b2b2b","name":"user"}}],"text":"Redis 的服务器周期性操作函数 serverCron 默认每隔 100 毫秒执行一次，该函数用于对正在运行的服务器进行维护，它的其中一项工作就是检查 save 选项所设置的保存条件是否已经满足，如果满足的话就执行 BGSAVE 命令。Redis serverCron 源码解析如下：","attrs":{}}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/44/446e4e4b221984030695e5a8137c055f.png","alt":null,"title":"","style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":"","fromPaste":false,"pastePass":false}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#2b2b2b","name":"user"}}],"text":"程序会遍历并检查 saveparams 数组中的所有保存条件，只要有任意一个条件被满足，服务器就会执行 BGSAVE 命令。下面是 rdbSaveBackground 的源码流程：","attrs":{}}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/c3/c34d14687753cc2042e6420a9f77a6ce.png","alt":null,"title":"","style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":"","fromPaste":false,"pastePass":false}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#40B8FA","name":"user"}}],"text":"RDB 文件结构","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#2b2b2b","name":"user"}}],"text":"下图展示了一个完整 RDB 文件所包含的各个部分。","attrs":{}}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/ea/eae116936ace09485566ca5e0cbdb44f.png","alt":"","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#2b2b2b","name":"user"}}],"text":"redis 文件的最开头是 ","attrs":{}},{"type":"text","marks":[{"type":"color","attrs":{"color":"#3594F7","name":"user"}},{"type":"strong","attrs":{}}],"text":"「REDIS」","attrs":{}},{"type":"text","text":" 部分，这个部分的长度是 5 字节，保存着 “REDIS” 五个字符。通过这五个字符，程序可以在载入文件时，快速检查所载入的文件是否时 RDB 文件。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#3594F7","name":"user"}},{"type":"strong","attrs":{}}],"text":"「db_version」","attrs":{}},{"type":"text","text":" 长度为 4 字节，他的值时一个字符串表示的整数，这个整数记录了 RDB 文件的版本号，比如 “0006” 就代表 RDB 文件的版本为第六版。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#3594F7","name":"user"}},{"type":"strong","attrs":{}}],"text":"「database」","attrs":{}},{"type":"text","text":" 部分包含着零个或任意多个数据库，以及各个数据库中的键值对数据：","attrs":{}}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#595959","name":"user"}}],"text":"如果服务器的数据库状态为空(所有数据库都是空的)，那么这个部分也为空，长度为 0 字节。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#595959","name":"user"}}],"text":"如果服务器的数据库状态为非空(有至少一个数据库非空)，那么这个部分也为非空，根据数据库所保存键值对的数量、类型和内容不同，这个部分的长度也会有所不同。","attrs":{}}]}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#3594F7","name":"user"}},{"type":"strong","attrs":{}}],"text":"「EOF」","attrs":{}},{"type":"text","text":" 常量的长度为 1 字节，这个常量标志着 RDB 文件正文内容的结束，当读入程序遇到这个值后，他知道所有数据库的所有键值对已经载入完毕了。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#3594F7","name":"user"}},{"type":"strong","attrs":{}}],"text":"「check_sum」","attrs":{}},{"type":"text","text":" 是一个 8 字节长的无符号整数，保存着一个校验和，这个校验和时程序通过对 REDIS、db_version、database、EOF 四个部分的内容进行计算得出的。服务器在载入 RDB 文件时，会将载入数据所计算出的校验和与 check_sum 所记录的校验和进行对比，以此来检查 RDB 是否有出错或者损坏的情况。举个例子：下图是一个 0 号数据库和 3 号数据库的 RDB 文件。第一个就是 “REDIS” 表示是一个 RDB 文件，之后的 “0006” 表示这是第六版的 REDIS 文件，然后是两个数据库，之后就是 EOF 结束标识符，最后就是 check_sum。","attrs":{}}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/61/612d76eb9de49a81d1ad3af45b5ffe5e.png","alt":"","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#40B8FA","name":"user"}}],"text":"AOF 持久化","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#40B8FA","name":"user"}}],"text":"什么是 AOF 持久化","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#2b2b2b","name":"user"}}],"text":"AOF持久化方式记录每次对服务器写的操作,当服务器重启的时候会重新执行这些命令来恢复原始的数据,AOF命令以redis协议追加保存每次写的操作到文件末尾.Redis还能对AOF文件进行后台重写,使得AOF文件的体积不至于过大.","attrs":{}}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/25/258888ba9a9ea1f0112c6e93999e8480.png","alt":"","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#40B8FA","name":"user"}}],"text":"AOF 的优点？","attrs":{}}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#595959","name":"user"}}],"text":"使用AOF 会让你的Redis更加耐久: 你可以使用不同的fsync策略：无fsync,每秒fsync,每次写的时候fsync.使用默认的每秒fsync策略,Redis的性能依然很好(fsync是由后台线程进行处理的,主线程会尽力处理客户端请求),一旦出现故障，你最多丢失1秒的数据.","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#595959","name":"user"}}],"text":"AOF文件是一个只进行追加的日志文件,所以不需要写入seek,即使由于某些原因(磁盘空间已满，写的过程中宕机等等)未执行完整的写入命令,你也也可使用redis-check-aof工具修复这些问题.","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#595959","name":"user"}}],"text":"Redis 可以在 AOF 文件体积变得过大时，自动地在后台对 AOF 进行重写： 重写后的新 AOF 文件包含了恢复当前数据集所需的最小命令集合。 整个重写操作是绝对安全的，因为 Redis 在创建新 AOF 文件的过程中，会继续将命令追加到现有的 AOF 文件里面，即使重写过程中发生停机，现有的 AOF 文件也不会丢失。 而一旦新 AOF 文件创建完毕，Redis 就会从旧 AOF 文件切换到新 AOF 文件，并开始对新 AOF 文件进行追加操作。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#595959","name":"user"}}],"text":"AOF 文件有序地保存了对数据库执行的所有写入操作， 这些写入操作以 Redis 协议的格式保存， 因此 AOF 文件的内容非常容易被人读懂， 对文件进行分析（parse）也很轻松。 导出（export） AOF 文件也非常简单： 举个例子， 如果你不小心执行了 FLUSHALL 命令， 但只要 AOF 文件未被重写， 那么只要停止服务器， 移除 AOF 文件末尾的 FLUSHALL 命令， 并重启 Redis ， 就可以将数据集恢复到 FLUSHALL 执行之前的状态。","attrs":{}}]}]}],"attrs":{}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#40B8FA","name":"user"}}],"text":"AOF 的缺点？","attrs":{}}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#595959","name":"user"}}],"text":"对于相同的数据集来说，AOF 文件的体积通常要大于 RDB 文件的体积。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#595959","name":"user"}}],"text":"根据所使用的 fsync 策略，AOF 的速度可能会慢于 RDB 。 在一般情况下， 每秒 fsync 的性能依然非常高， 而关闭 fsync 可以让 AOF 的速度和 RDB 一样快， 即使在高负荷之下也是如此。 不过在处理巨大的写入载入时，RDB 可以提供更有保证的最大延迟时间（latency）。","attrs":{}}]}]}],"attrs":{}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#40B8FA","name":"user"}}],"text":"AOF持久化的实现","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#2b2b2b","name":"user"}}],"text":"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。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#2b2b2b","name":"user"}}],"text":"命令追加","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#2b2b2b","name":"user"}}],"text":"当 AOF 持久化功能处于打开状态时，服务器在执行完一个写命令之后，会以协议格式将被执行的写命令追加到服务器状态的 aof_buf 缓冲区的末尾。","attrs":{}}]},{"type":"codeblock","attrs":{"lang":"cpp"},"content":[{"type":"text","text":"struct redisServer {\n  // ...\n  // AOF 缓冲区  \n  sds aof_buf;\n  \n  // ..\n};\n","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#2b2b2b","name":"user"}}],"text":"如果客户端向服务器发送以下命令：","attrs":{}}]},{"type":"codeblock","attrs":{"lang":"cpp"},"content":[{"type":"text","text":"> set KEY VALUE\nOK\n","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#2b2b2b","name":"user"}}],"text":"那么服务器在执行这个 set 命令之后，会将以下协议内容追加到 aof_buf 缓冲区的末尾；","attrs":{}}]},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"*3\\r\\n$3\\r\\nSET\\r\\n$3\\r\\nKEY\\r\\n$5\\r\\nVALUE\\r\\n\n","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#2b2b2b","name":"user"}}],"text":"AOF 文件的写入与同步","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#2b2b2b","name":"user"}}],"text":"  Redis的服务器进程就是一个事件循环（loop），这个循环中的文件事件负责接收客户端的命令请求，以及向客户端发送命令回复，而时间事件则负责执行像 serverCron 函数这样需要定时运行的函数。  因为服务器在处理文件事件时可能会执行写命令，使得一些内容被追加到aof_buf缓冲区里面，所以在服务器每次结束一个事件循环之前，它都会调用 ","attrs":{}},{"type":"text","marks":[{"type":"color","attrs":{"color":"#3594F7","name":"user"}},{"type":"strong","attrs":{}}],"text":"「flushAppendOnlyFile」","attrs":{}},{"type":"text","text":" 函数，考虑是否需要将aof_buf缓冲区中的内容写入和保存到AOF文件里面，这个过程可以用以下伪代码表示：","attrs":{}}]},{"type":"codeblock","attrs":{"lang":"cpp"},"content":[{"type":"text","text":"def eventLoop():\n  while True:\n  \n  #处理文件事件，接收命令请求以及发送命令回复\n  #处理命令请求时可能会有新内容被追加到 aof_buf缓冲区中\n  processFileEvents()\n  \n  #处理时间事件\n  processTimeEvents()\n  \n  #考虑是否要将 aof_buf中的内容写入和保存到 AOF文件里面\n  flushAppendOnlyFile()\n","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#2b2b2b","name":"user"}}],"text":"flushAppendOnlyFile函数的行为由服务器配置的 ","attrs":{}},{"type":"text","marks":[{"type":"color","attrs":{"color":"#3594F7","name":"user"}},{"type":"strong","attrs":{}}],"text":"「appendfsync」","attrs":{}},{"type":"text","text":" 选项的值来决定，各个不同值产生的行为如下表所示。","attrs":{}}]},{"type":"embedcomp","attrs":{"type":"table","data":{"content":"

appendfsync 选项的值	flushAppendOnlyFile 函数的行为
always	将 aof_buf 缓冲区中的所有内容写入并同步到 AOF 文件
everysec	将 aof_buf 缓冲区中的所有内容写入到 AOF 文件，如果上次同步 AOF 文件的时间距离现在超过一秒钟，那么再次对 AOF 文件进行同步，并且这个同步操作是由一个线程专门负责执行的
no	将 aof_buf 缓冲区中的所有内容写入到 AOF 文件，但并不对 AOF 文件进行同步，何时同步由操作系统来决定

"}}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#2b2b2b","name":"user"}}],"text":"如果用户没有主动为appendfsync选项设置值，那么appendfsync选项的默认值为everysec。写到这里有的小伙伴可能会对上面说的写入和同步含义弄混，这里说一下：","attrs":{}},{"type":"text","marks":[{"type":"color","attrs":{"color":"#3594F7","name":"user"}},{"type":"strong","attrs":{}}],"text":"「写入：将 aof_buf 中的数据写入到 AOF 文件中。」「同步：调用 fsync 以及 fdatasync 函数，将 AOF 文件中的数据保存到磁盘中。」","attrs":{}},{"type":"text","text":"通俗地讲就是，你要往一个文件写东西，写的过程就是写入，而同步则是将文件保存，数据落到磁盘上。大家之前看文章的时候是不是大多都说 AOF 最多丢失一秒钟的数据，那是因为 redis AOF 默认是 everysec 策略，这个策略每秒执行一次，所以 AOF 持久化最多丢失一秒钟的数据。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#2b2b2b","name":"user"}}],"text":"AOF 文件的载入与数据还原","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#2b2b2b","name":"user"}}],"text":"因为AOF文件里面包含了重建数据库状态所需的所有写命令，所以服务器只要读入并重新执行一遍AOF文件里面保存的写命令，就可以还原服务器关闭之前的数据库状态。 Redis读取AOF文件并还原数据库状态的详细步骤如下：","attrs":{}}]},{"type":"numberedlist","attrs":{"start":null,"normalizeStart":1},"content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#595959","name":"user"}}],"text":"创建一个不带网络连接的伪客户端（fake client）：因为Redis的命令只能在客户端上 下文中执行，而载入AOF文件时所使用的命令直接来源于AOF文件而不是网络连接，所以服 务器使用了一个没有网络连接的伪客户端来执行AOF文件保存的写命令，伪客户端执行命令 的效果和带网络连接的客户端执行命令的效果完全一样。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#595959","name":"user"}}],"text":"从AOF文件中分析并读取出一条写命令。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#595959","name":"user"}}],"text":"使用伪客户端执行被读出的写命令。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":4,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#595959","name":"user"}}],"text":"一直执行步骤2和步骤3，直到AOF文件中的所有写命令都被处理完毕为止。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":5,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":6,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#595959","name":"user"}}],"text":"当完成以上步骤之后，AOF文件所保存的数据库状态就会被完整地还原出来，整个过程 如下图所示。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":7,"align":null,"origin":null}}]}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/54/5492a19465f8f3c4743b4a4e1554c24d.png","alt":"","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"numberedlist","attrs":{"start":null,"normalizeStart":1},"content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null}}]}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#2b2b2b","name":"user"}}],"text":"AOF 重写","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#2b2b2b","name":"user"}}],"text":"因为AOF持久化是通过保存被执行的写命令来记录数据库状态的，所以随着服务器运行 时间的流逝，AOF文件中的内容会越来越多，文件的体积也会越来越大，如果不加以控制的 话，体积过大的AOF文件很可能对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文件来进行数据还原所需的时间就越多。如 客户端执行了以下命令是：","attrs":{}}]},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"> rpush list \"A\" \"B\"\nOK\n> rpush list \"C\"\nOK\n> rpush list \"D\"\nOK\n> rpush list \"E\" \"F\"\nOK\n","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#2b2b2b","name":"user"}}],"text":"  那么光是为了记录这个list键的状态，AOF文件就需要保存四条命令。  对于实际的应用程度来说，写命令执行的次数和频率会比上面的简单示例要高得多，所 以造成的问题也会严重得多。 为了解决AOF文件体积膨胀的问题，Redis提供了AOF文件重写（rewrite）功能。通过该 功能，Redis服务器可以创建一个新的AOF文件来替代现有的AOF文件，新旧两个AOF文件所 保存的数据库状态相同，但新AOF文件不会包含任何浪费空间的冗余命令，所以新AOF文件 的体积通常会比旧AOF文件的体积要小得多。 在接下来的内容中，我们将介绍AOF文件重写的实现原理，以及BGREWEITEAOF命令 的实现原理。  虽然Redis将生成新AOF文件替换旧AOF文件的功能命名为“AOF文件重写”，但实际上， AOF文件重写并不需要对现有的AOF文件进行任何读取、分析或者写入操作，这个功能是通 过读取服务器当前的数据库状态来实现的。  就像上面的情况，服务器完全可以将这六条命令合并成一条。","attrs":{}}]},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"> rpush list \"A\" \"B\" \"C\" \"D\" \"E\" \"F\"\n","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#2b2b2b","name":"user"}}],"text":"  除了上面列举的列表键之外，其他所有类型的键都可以用同样的方法去减少 AOF文件中的命令数量。首先从数据库中读取键现在的值，然后用一条命令去记录键值对，代替之前记录这个键值对的多条命令，这就是AOF重写功能的实现原理。  在实际中，为了避免在执行命令时造成客户端输入缓冲区溢出，重写程序在处理列表、 哈希表、集合、有序集合这四种可能会带有多个元素的键时，会先检查键所包含的元素数 量，如果元素的数量超过了redis.h/REDIS_AOF_REWRITE_ITEMS_PER_CMD常量的值，那 么重写程序将使用多条命令来记录键的值，而不单单使用一条命令。 在目前版本中，REDIS_AOF_REWRITE_ITEMS_PER_CMD常量的值为64，这也就是 说，如果一个集合键包含了超过64个元素，那么重写程序会用多条SADD命令来记录这个集 合，并且每条命令设置的元素数量也为64个。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#2b2b2b","name":"user"}}],"text":"AOF 后台重写","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#2b2b2b","name":"user"}}],"text":"  AOF 重写会执行大量的写操作，这样会影响主线程，所以redis AOF 重写放到了子进程去执行。这样可以达到两个目的：","attrs":{}}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#595959","name":"user"}}],"text":"子进程进行AOF重写期间，服务器进程（父进程）可以继续处理命令请求。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#595959","name":"user"}}],"text":"子进程带有服务器进程的数据副本，使用子进程而不是线程，可以在避免使用锁的情况 下，保证数据的安全性。","attrs":{}}]}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#2b2b2b","name":"user"}}],"text":"但是有一个问题，当子进程重写数据时，主进程依然在处理新的数据，这也就会造成数据不一致情况。","attrs":{}},{"type":"text","marks":[{"type":"color","attrs":{"color":"#3594F7","name":"user"}},{"type":"strong","attrs":{}}],"text":"「为了解决这种数据不一致问题，Redis服务器设置了一个AOF重写缓冲区，这个缓冲区在 服务器创建子进程之后开始使用，当Redis服务器执行完一个写命令之后，它会同时将这个写 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