redis学习笔记——RDB和AOF持久化二

上一篇对RDB的源码分析是比较多的，但是AOF持久化执行进行了一些理论上的分析和概念的说明。本来想自己偷一些懒，将上篇文章中最后所给链接的AOF实现代码随便过一过算了，后来也就是在过的过程中发现自己这也看不懂那也看不懂才知道AOF的重要性和难度。

后来又花了不少时间查阅资料、结合源代码分析，对AOF的大概执行过程有了更深一些的了解，现在就将自己的理解和大家进行分享。其中肯定有理解不正确的地方，还望大神们能给予指正。

AOF相关配置项

首先我们看一下redis.conf里的关于AOF的配置选项：
Appendonly(yes,no)——是否开启AOF持久化
Appendfilename(log/appendonly.aof)——AOF日志文件
Appendfsync(always,everysec,no)——AOF日志文件同步的频率，always代表每次写都进行fsync，everysec每秒钟一次，no不主动fsync，由OS自己来完成。
no-appendfsync-on-rewrite(yes,no)——进行rewrite时，是否需要fsync
auto-aof-rewrite-percentage(100)——当AOF文件增长了这个比例（这里是增加了一倍），则后台rewrite自动运行
auto-aof-rewrite-min-size(64mb)——进行后面rewrite要求的最小AOF文件大小。这两个选项共同决定了后面rewrite进程是否到达运行的时机

通过上面的选项我们可以知道redis有三个AOF处理流程：

• 每次更新操作进行的AOF写操作（涉及同步频率）；
• Rewrite，当满足auto-aof-rewrite-percentage，auto-aof-rewrite-min-size时后面自动运行rewrite操作；
• Rewrite，当收到bgrewriteaof客户端命令时，马上运行后面rewrite操作。

注：当某个key过期的时候也会写AOF,其实它跟第一种很类似，也就是DEL操作。

在redis的较新版本中（不知道从哪个版本开始）增加了两个新的子进程：

• REDIS_BIO_CLOSE_FILE，负责所有的close file操作
• REDIS_BIO_AOF_FSYNC，负责fsync操作

因为这两个操作都可能会引起阻塞，如果在主线程中完成的话，会影响系统对事件的响应，所以这里统一由相应的子线程来完成，每个子线程都有一个自己的bio_jobs list，用来保存需要的处理的job任务。其相应的代码在bio.c（线程处理函数为bioProcessBackgroundJobs）里，这两个线程在initServer时创建bioInit()。

void initServer() {
//...
// 初始化 BIO 系统
    bioInit();
}

AOF的处理流程

　　1.每次更新操作进行的AOF写操作（涉及同步频率）

主要涉及的配置是：Appendfsync（AOF日志文件同步的频率），no-appendfsync-on-rewrite（进行rewrite时，是否需要fsync），该操作的入口在redis.c。

void call(redisClient *c, int flags) {
... 
// 保留旧 dirty 计数器值
    dirty = server.dirty;
    // 计算命令开始执行的时间
    start = ustime();
    // 执行实现函数
    c->cmd->proc(c);
    // 计算命令执行耗费的时间
    duration = ustime()-start;
    // 计算命令执行之后的 dirty 值
    dirty = server.dirty-dirty;
    ....
    /* Propagate the command into the AOF and replication link */
    // 将命令复制到 AOF 和 slave 节点
    if (flags & REDIS_CALL_PROPAGATE) {
        int flags = REDIS_PROPAGATE_NONE;
        // 强制 REPL 传播
        if (c->flags & REDIS_FORCE_REPL) flags |= REDIS_PROPAGATE_REPL;

        // 强制 AOF 传播
        if (c->flags & REDIS_FORCE_AOF) flags |= REDIS_PROPAGATE_AOF;

        // 如果数据库有被修改，那么启用 REPL 和 AOF 传播
        if (dirty)
            flags |= (REDIS_PROPAGATE_REPL | REDIS_PROPAGATE_AOF);

        if (flags != REDIS_PROPAGATE_NONE)
            propagate(c->cmd,c->db->id,c->argv,c->argc,flags);
    }
    ...
}

我们再来看一下propagate的实现:

void propagate(struct redisCommand *cmd, int dbid, robj **argv, int argc,
               int flags)
{
    // 传播到 AOF
    if (server.aof_state != REDIS_AOF_OFF && flags & REDIS_PROPAGATE_AOF)
        feedAppendOnlyFile(cmd,dbid,argv,argc);

    // 传播到 slave
    if (flags & REDIS_PROPAGATE_REPL)
        replicationFeedSlaves(server.slaves,dbid,argv,argc);
}

我们再来看一下feedAppendOnlyFile的实现:

void feedAppendOnlyFile(struct redisCommand…{
if (dictid != server.aof_selected_db) {//当前操作的db与上一次不一样，所以要重新写一个新的select db命令，当rewrite的时候也会把appendseldb置为-1
        char seldb[64];
        snprintf(seldb,sizeof(seldb),"%d",dictid);
        buf = sdscatprintf(buf,"*2
$6
SELECT
$%lu
%s
",
            (unsigned long)strlen(seldb),seldb);
        server.aof_selected_db = dictid;
 }
…

buf = catAppendOnlyGenericCommand(buf,argc,argv); //转换为标准命令格式
server.aofbuf = sdscatlen(server.aofbuf,buf,sdslen(buf)); //将命令写到aofbuf,这个buf会在serverCron当Appendfsync到满足时fsync到文件
if (server.bgrewritechildpid != -1) //如果有bgrewrite子进程的话，则也必须把该命令保存到bgrewritebuf,以便在子进程结束时,把新的变更追加到rewrite后的文件
    server.bgrewritebuf = sdscatlen(server.bgrewritebuf,buf,sdslen(buf));
…
}

可以看到到上面AOF操作也只是写到buf中，并没有将其写到文件中，下面我们将查看写到文件中的过程。通过查看代码我们可以知道flushAppendOnlyFile()函数是进行真正的写入文件操作。另外我们可以知道该函数会在beforeSleep及serverCron中调用。其中beforeSleep是aeMain循环，每次进行事件处理前必须调用一次:

void aeMain(aeEventLoop *eventLoop) {
    eventLoop->stop = 0;
    while (!eventLoop->stop) {
        if (eventLoop->beforesleep != NULL)
            eventLoop->beforesleep(eventLoop);
        aeProcessEvents(eventLoop, AE_ALL_EVENTS);
    }
}

/* This function gets called every time Redis is entering the
 * main loop of the event driven library, that is, before to sleep
 * for ready file descriptors. */
// 每次处理事件之前执行
void beforeSleep(struct aeEventLoop *eventLoop) {
    ...
    /* Write the AOF buffer on disk */
    // 将 AOF 缓冲区的内容写入到 AOF 文件
    flushAppendOnlyFile(0);
    ...
}

int serverCron(struct aeEventLoop *eventLoop, long long id, void *clientData) {
    ...
     // 根据 AOF 政策，
    // 考虑是否需要将 AOF 缓冲区中的内容写入到 AOF 文件中
    /* AOF postponed flush: Try at every cron cycle if the slow fsync
     * completed. */
    if (server.aof_flush_postponed_start) flushAppendOnlyFile(0);
    ...
}

下面我们来看一下该函数flushAppendOnlyFile的实现

/* Write the append only file buffer on disk.
 *
 * 将 AOF 缓存写入到文件中。
 *
 * Since we are required to write the AOF before replying to the client,
 * and the only way the client socket can get a write is entering when the
 * the event loop, we accumulate all the AOF writes in a memory
 * buffer and write it on disk using this function just before entering
 * the event loop again.
 *
 * 因为程序需要在回复客户端之前对 AOF 执行写操作。
 * 而客户端能执行写操作的唯一机会就是在事件 loop 中，
 * 因此，程序将所有 AOF 写累积到缓存中，
 * 并在重新进入事件 loop 之前，将缓存写入到文件中。
 *
 * About the 'force' argument:
 *
 * 关于 force 参数：
 *
 * When the fsync policy is set to 'everysec' we may delay the flush if there
 * is still an fsync() going on in the background thread, since for instance
 * on Linux write(2) will be blocked by the background fsync anyway.
 *
 * 当 fsync 策略为每秒钟保存一次时，如果后台线程仍然有 fsync 在执行，
 * 那么我们可能会延迟执行冲洗（flush）操作，
 * 因为 Linux 上的 write(2) 会被后台的 fsync 阻塞。
 *
 * When this happens we remember that there is some aof buffer to be
 * flushed ASAP, and will try to do that in the serverCron() function.
 *
 * 当这种情况发生时，说明需要尽快冲洗 aof 缓存，
 * 程序会尝试在 serverCron() 函数中对缓存进行冲洗。
 *
 * However if force is set to 1 we'll write regardless of the background
 * fsync. 
 *
 * 不过，如果 force 为 1 的话，那么不管后台是否正在 fsync ，
 * 程序都直接进行写入。
 */
#define AOF_WRITE_LOG_ERROR_RATE 30 /* Seconds between errors logging. */
void flushAppendOnlyFile(int force) {
    ssize_t nwritten;
    int sync_in_progress = 0;

    // 缓冲区中没有任何内容，直接返回
    if (sdslen(server.aof_buf) == 0) return;

    // 策略为每秒 FSYNC 
    if (server.aof_fsync == AOF_FSYNC_EVERYSEC)
        // 是否有 SYNC 正在后台进行？
        sync_in_progress = bioPendingJobsOfType(REDIS_BIO_AOF_FSYNC) != 0;

    // 每秒 fsync ，并且强制写入为假
    if (server.aof_fsync == AOF_FSYNC_EVERYSEC && !force) {

        /* With this append fsync policy we do background fsyncing.
         *
         * 当 fsync 策略为每秒钟一次时， fsync 在后台执行。
         *
         * If the fsync is still in progress we can try to delay
         * the write for a couple of seconds. 
         *
         * 如果后台仍在执行 FSYNC ，那么我们可以延迟写操作一两秒
         * （如果强制执行 write 的话，服务器主线程将阻塞在 write 上面）
         */
        if (sync_in_progress) {

            // 有 fsync 正在后台进行 。。。

            if (server.aof_flush_postponed_start == 0) {
                /* No previous write postponinig, remember that we are
                 * postponing the flush and return. 
                 *
                 * 前面没有推迟过 write 操作，这里将推迟写操作的起始时间记录下来
                 * 然后就返回，不执行 write 或者 fsync
                 */
                server.aof_flush_postponed_start = server.unixtime;
                return;

            } else if (server.unixtime - server.aof_flush_postponed_start < 2) {
                /* We were already waiting for fsync to finish, but for less
                 * than two seconds this is still ok. Postpone again. 
                 *
                 * 如果之前已经因为 fsync 而推迟了 write 操作
                 * 但是推迟的时间不超过 2 秒，那么直接返回
                 * 不执行 write 或者 fsync
                 */
                return;

            }

            /* Otherwise fall trough, and go write since we can't wait
             * over two seconds. 
             *
             * 如果后台还有 fsync 在执行，并且 write 已经推迟 >= 2 秒
             * 那么执行写操作（write 将被阻塞）
             */
            server.aof_delayed_fsync++;
            redisLog(REDIS_NOTICE,"Asynchronous AOF fsync is taking too long (disk is busy?). Writing the AOF buffer without waiting for fsync to complete, this may slow down Redis.");
        }
    }

    /* If you are following this code path, then we are going to write so
     * set reset the postponed flush sentinel to zero. 
     *
     * 执行到这里，程序会对 AOF 文件进行写入。
     *
     * 清零延迟 write 的时间记录
     */
    server.aof_flush_postponed_start = 0;

    /* We want to perform a single write. This should be guaranteed atomic
     * at least if the filesystem we are writing is a real physical one.
     *
     * 执行单个 write 操作，如果写入设备是物理的话，那么这个操作应该是原子的
     *
     * While this will save us against the server being killed I don't think
     * there is much to do about the whole server stopping for power problems
     * or alike 
     *
     * 当然，如果出现像电源中断这样的不可抗现象，那么 AOF 文件也是可能会出现问题的
     * 这时就要用 redis-check-aof 程序来进行修复。
     */
    nwritten = write(server.aof_fd,server.aof_buf,sdslen(server.aof_buf));
    if (nwritten != (signed)sdslen(server.aof_buf)) {//写入文件有错

        static time_t last_write_error_log = 0;
        int can_log = 0;

        /* Limit logging rate to 1 line per AOF_WRITE_LOG_ERROR_RATE seconds. */
        // 将日志的记录频率限制在每行 AOF_WRITE_LOG_ERROR_RATE 秒
        if ((server.unixtime - last_write_error_log) > AOF_WRITE_LOG_ERROR_RATE) {
            can_log = 1;
            last_write_error_log = server.unixtime;
        }

        /* Lof the AOF write error and record the error code. */
        // 如果写入出错，那么尝试将该情况写入到日志里面
        if (nwritten == -1) {
            if (can_log) {
                redisLog(REDIS_WARNING,"Error writing to the AOF file: %s",
                    strerror(errno));
                server.aof_last_write_errno = errno;
            }
        } else {
            if (can_log) {
                redisLog(REDIS_WARNING,"Short write while writing to "
                                       "the AOF file: (nwritten=%lld, "
                                       "expected=%lld)",
                                       (long long)nwritten,
                                       (long long)sdslen(server.aof_buf));
            }

            // 尝试移除新追加的不完整内容
            if (ftruncate(server.aof_fd, server.aof_current_size) == -1) {
                if (can_log) {
                    redisLog(REDIS_WARNING, "Could not remove short write "
                             "from the append-only file.  Redis may refuse "
                             "to load the AOF the next time it starts.  "
                             "ftruncate: %s", strerror(errno));
                }
            } else {
                /* If the ftrunacate() succeeded we can set nwritten to
                 * -1 since there is no longer partial(部分的，局部的) data into the AOF. */
                nwritten = -1;
            }
            server.aof_last_write_errno = ENOSPC;
        }

        /* Handle the AOF write error. */
        // 处理写入 AOF 文件时出现的错误
        if (server.aof_fsync == AOF_FSYNC_ALWAYS) {
            /* We can't recover when the fsync policy is ALWAYS since the
             * reply for the client is already in the output buffers, and we
             * have the contract with the user that on acknowledged write data
             * is synched on disk. */
            //当fsync是ALWAYS时，那么如果出错我们是不可能进行恢复的，因为尽管出错，我们对用户的回复已经
            //到达了输出缓冲区，并且我们还向用户说明(set sadd等操作的)写数据已经写到了磁盘
            redisLog(REDIS_WARNING,"Can't recover from AOF write error when the AOF fsync policy is 'always'. Exiting...");
            exit(1);
        } else {
            /* Recover from failed write leaving data into the buffer. However
             * set an error to stop accepting writes as long as the error
             * condition is not cleared. */
            server.aof_last_write_status = REDIS_ERR;

            /* Trim the sds buffer if there was a partial write, and there
             * was no way to undo it with ftruncate(2). */
            //如果这是局部写的话（我靠，我也翻译不好），那就缩减sds buffer(aof_buffer)的大小
            if (nwritten > 0) {
                server.aof_current_size += nwritten;
                sdsrange(server.aof_buf,nwritten,-1);
            }
            return; /* We'll try again on the next call... */
        }
    } else {//写入文件没错
        /* Successful write(2). If AOF was in error state, restore the
         * OK state and log the event. */
        // 写入成功，更新最后写入状态
        if (server.aof_last_write_status == REDIS_ERR) {
            redisLog(REDIS_WARNING,
                "AOF write error looks solved, Redis can write again.");
            server.aof_last_write_status = REDIS_OK;
        }
    }

    // 更新写入后的 AOF 文件大小
    server.aof_current_size += nwritten;

    /* Re-use AOF buffer when it is small enough. The maximum comes from the
     * arena size of 4k minus some overhead (but is otherwise arbitrary). 
     *
     * 如果 AOF 缓存的大小足够小的话，那么重用这个缓存，
     * 否则的话，释放 AOF 缓存。
     * sdsavail(server.aof_buf)返回 aof_buf 可用空间的长度
     * sdslen(server.aof_buf)返回 aof_buf 实际保存的字符串的长度
     */
    if ((sdslen(server.aof_buf)+sdsavail(server.aof_buf)) < 4000) {
        // 清空缓存中的内容，等待重用
        sdsclear(server.aof_buf);
    } else {
        // 释放缓存
        sdsfree(server.aof_buf);
        server.aof_buf = sdsempty();
    }

    /* Don't fsync if no-appendfsync-on-rewrite is set to yes and there are
     * children doing I/O in the background. 
     *
     * 如果 no-appendfsync-on-rewrite 选项为开启状态，
     * 并且有 BGSAVE 或者 BGREWRITEAOF 正在进行的话，
     * 那么不执行 fsync 
     */
    if (server.aof_no_fsync_on_rewrite &&
        (server.aof_child_pid != -1 || server.rdb_child_pid != -1))
            return;

    /* Perform the fsync if needed. */

    // 总是执行 fsnyc
    if (server.aof_fsync == AOF_FSYNC_ALWAYS) {
        /* aof_fsync is defined as fdatasync() for Linux in order to avoid
         * flushing metadata. */
        aof_fsync(server.aof_fd); /* Let's try to get this data on the disk */
        // 更新最后一次执行 fsnyc 的时间
        server.aof_last_fsync = server.unixtime;
    // 策略为每秒 fsnyc ，并且距离上次 fsync 已经超过 1 秒
    } else if ((server.aof_fsync == AOF_FSYNC_EVERYSEC &&
                server.unixtime > server.aof_last_fsync)) {
        // 放到后台执行
        if (!sync_in_progress) aof_background_fsync(server.aof_fd);
        // 更新最后一次执行 fsync 的时间
        server.aof_last_fsync = server.unixtime;
    }

    // 其实上面无论执行 if 部分还是 else 部分都要更新 fsync 的时间
    // 可以将代码挪到下面来
    // server.aof_last_fsync = server.unixtime;
}

 通过上面的介绍我们可以知道即使Appendfsync设置为alway，并不是每次执行完一条更新命令就直接写（write+fsync）aof file，这个过程（write+fsync）会被推迟到事件处理流程结束后beforeSleep后进行(一个疑问先写到server.aofbuf，然后再写到数据文件，过程中如果crash会不会丢数据呢？ 答案是：不会，因为在一次事件处理结束之后会调用beforeSleep进行flash，而它也是在下一次事件处理之前完成的,即只有在同步到文件之后才会给客户端回复成功与否)；如果在beforeSleep时已经有fsync job在等待fsync线程处理(只有一个aof fd,之前还在想为什么它不能再被放到list里)，if (server.appendfsync == APPENDFSYNC_EVERYSEC && !force) && if (sync_in_progress)，则该次的请求会被标志为server.aof_flush_postponed_start，那么在调用serverCron时会再次调用flushAppendOnlyFile，看是否现在能够进行write并且把该job提交给fsync线程,或者如果已经等待超过2s,则给出一个系统提示。[同样的貌似everysec，也并不是真正的每1s fsync一次]

　　2.后面自动运行rewrite

该操作涉及的配置：auto-aof-rewrite-percentage，auto-aof-rewrite-min-size。
该过程是在serverCron里判断，是满足到达运行bgrewrite的时机：

int serverCron(struct aeEventLoop *eventLoop, long long id, void *clientData){
    ...
     /* Start a scheduled AOF rewrite if this was requested by the user while
     * a BGSAVE was in progress. */
    // 如果 BGSAVE 和 BGREWRITEAOF 都没有在执行
    // 并且有一个 BGREWRITEAOF 在等待，那么执行 BGREWRITEAOF
    if (server.rdb_child_pid == -1 && server.aof_child_pid == -1 &&
        server.aof_rewrite_scheduled)
    {
        rewriteAppendOnlyFileBackground();
    }

    /* Check if a background saving or AOF rewrite in progress terminated. */
    // 检查 BGSAVE 或者 BGREWRITEAOF 是否已经执行完毕
    if (server.rdb_child_pid != -1 || server.aof_child_pid != -1) {
        int statloc;
        pid_t pid;

        // 接收子进程发来的信号，非阻塞
        if ((pid = wait3(&statloc,WNOHANG,NULL)) != 0) {
            int exitcode = WEXITSTATUS(statloc);
            int bysignal = 0;
            
            if (WIFSIGNALED(statloc)) bysignal = WTERMSIG(statloc);

            // BGSAVE 执行完毕
            if (pid == server.rdb_child_pid) {
                backgroundSaveDoneHandler(exitcode,bysignal);

            // BGREWRITEAOF 执行完毕
            } else if (pid == server.aof_child_pid) {
                backgroundRewriteDoneHandler(exitcode,bysignal);

            } else {
                redisLog(REDIS_WARNING,
                    "Warning, detected child with unmatched pid: %ld",
                    (long)pid);
            }
            updateDictResizePolicy();
        }
    } else {

        /* If there is not a background saving/rewrite in progress check if
         * we have to save/rewrite now */
        // 既然没有 BGSAVE 或者 BGREWRITEAOF 在执行，那么检查是否需要执行它们

        // 遍历所有保存条件，看是否需要执行 BGSAVE 命令
         for (j = 0; j < server.saveparamslen; j++) {
            struct saveparam *sp = server.saveparams+j;

            /* Save if we reached the given amount of changes,
             * the given amount of seconds, and if the latest bgsave was
             * successful or if, in case of an error, at least
             * REDIS_BGSAVE_RETRY_DELAY seconds already elapsed. */
            // 检查是否有某个保存条件已经满足了
            if (server.dirty >= sp->changes &&
                server.unixtime-server.lastsave > sp->seconds &&
                (server.unixtime-server.lastbgsave_try >
                 REDIS_BGSAVE_RETRY_DELAY ||
                 server.lastbgsave_status == REDIS_OK))
            {
                redisLog(REDIS_NOTICE,"%d changes in %d seconds. Saving...",
                    sp->changes, (int)sp->seconds);
                // 执行 BGSAVE
                rdbSaveBackground(server.rdb_filename);
                break;
            }
         }

         /* Trigger an AOF rewrite if needed */
        // 出发 BGREWRITEAOF
         if (server.rdb_child_pid == -1 &&
             server.aof_child_pid == -1 &&
             server.aof_rewrite_perc &&
             // AOF 文件的当前大小大于执行 BGREWRITEAOF 所需的最小大小
             server.aof_current_size > server.aof_rewrite_min_size)
         {
            // 上一次完成 AOF 写入之后，AOF 文件的大小
            long long base = server.aof_rewrite_base_size ?
                            server.aof_rewrite_base_size : 1;

            // AOF 文件当前的体积相对于 base 的体积的百分比
            long long growth = (server.aof_current_size*100/base) - 100;

            // 如果增长体积的百分比超过了 growth ，那么执行 BGREWRITEAOF
            if (growth >= server.aof_rewrite_perc) {
                redisLog(REDIS_NOTICE,"Starting automatic rewriting of AOF on %lld%% growth",growth);
                // 执行 BGREWRITEAOF
                rewriteAppendOnlyFileBackground();
            }
         }
    }
    ...
}

　　3. 客户端发送bgrewriteaof命令 

　　通过查找readonlyCommandTable表，我们可以看到当客户端发送bgrewriteaof命令过来的时候，服务器调用bgrewriteaofCommand函数来进行处理。该函数会判断当前是否已经有bgrewritechildpid存在，或者bgsavechildpid存在则标志server.aofrewrite_scheduled = 1,需要进行bgrewrite,但不是现在，而是在serverCron处理的时候。否则则直接调用rewriteAppendOnlyFileBackground，创建bgrewrite进程，进行rewrite操作。

rewriteAppendOnlyFileBackground实现如下：

/* This is how rewriting of the append only file in background works:
 * 
 * 以下是后台重写 AOF 文件（BGREWRITEAOF）的工作步骤：
 *
 * 1) The user calls BGREWRITEAOF
 *    用户调用 BGREWRITEAOF
 *
 * 2) Redis calls this function, that forks():
 *    Redis 调用这个函数，它执行 fork() ：
 *
 *    2a) the child rewrite the append only file in a temp file.
 *        子进程在临时文件中对 AOF 文件进行重写
 *
 *    2b) the parent accumulates differences in server.aof_rewrite_buf.
 *        父进程将新输入的写命令追加到 server.aof_rewrite_buf 中
 *
 * 3) When the child finished '2a' exists.
 *    当步骤 2a 执行完之后，子进程结束
 *
 * 4) The parent will trap the exit code, if it's OK, will append the
 *    data accumulated into server.aof_rewrite_buf into the temp file, and
 *    finally will rename(2) the temp file in the actual file name.
 *    The the new file is reopened as the new append only file. Profit!
 *
 *    父进程会捕捉子进程的退出信号，
 *    如果子进程的退出状态是 OK 的话，
 *    那么父进程将新输入命令的缓存追加到临时文件，
 *    然后使用 rename(2) 对临时文件改名，用它代替旧的 AOF 文件，
 *    至此，后台 AOF 重写完成。
 */
int rewriteAppendOnlyFileBackground(void) {
    pid_t childpid;
    long long start;

    // 已经有子进程在进行 AOF 重写了
    if (server.aof_child_pid != -1) return REDIS_ERR;

    // 记录 fork 开始前的时间，计算 fork 耗时用
    start = ustime();

    if ((childpid = fork()) == 0) {
        char tmpfile[256];

        /* Child */

        // 关闭监听(在我看来子进程完全复制了父进程的资源后也会有监听，所以需要关闭子进程监听的东西)
        closeListeningSockets(0);

        // 为进程设置名字，方便记认
        redisSetProcTitle("redis-aof-rewrite");

        // 创建临时文件，并进行 AOF 重写
        snprintf(tmpfile,256,"temp-rewriteaof-bg-%d.aof", (int) getpid());
        if (rewriteAppendOnlyFile(tmpfile) == REDIS_OK) {
            //脏数据，其实就是子进程消耗的内存大小
            //获取脏数据大小
            size_t private_dirty = zmalloc_get_private_dirty();
            //记录脏数据
            if (private_dirty) {
                redisLog(REDIS_NOTICE,
                    "AOF rewrite: %zu MB of memory used by copy-on-write",
                    private_dirty/(1024*1024));
            }
            // 发送重写成功信号
            exitFromChild(0);
        } else {
            // 发送重写失败信号
            exitFromChild(1);
        }
    } else {
        /* Parent */
        // 记录执行 fork 所消耗的时间
        server.stat_fork_time = ustime()-start;

        if (childpid == -1) {
            redisLog(REDIS_WARNING,
                "Can't rewrite append only file in background: fork: %s",
                strerror(errno));
            return REDIS_ERR;
        }

        redisLog(REDIS_NOTICE,
            "Background append only file rewriting started by pid %d",childpid);

        // 记录 AOF 重写的信息
        server.aof_rewrite_scheduled = 0;
        server.aof_rewrite_time_start = time(NULL);
        server.aof_child_pid = childpid;

        //更新rehash的(条件)，可以查看该函数的具体函数说明(这里是为了关闭rehash)
        updateDictResizePolicy();

        /* We set append_sel_db to -1 in order to force the next call to the
         * feedAppendOnlyFile() to issue a SELECT command, so the differences
         * accumulated by the parent into server.aof_rewrite_buf will start
         * with a SELECT statement and it will be safe to merge. 
         *
         * 将 aof_selected_db 设为 -1 ，
         * 强制让 feedAppendOnlyFile() 下次执行时引发一个 SELECT 命令，
         * 从而确保之后新添加的命令会设置到正确的数据库中
         */
        server.aof_selected_db = -1;
        //清空脚本缓存
        replicationScriptCacheFlush();
        return REDIS_OK;
    }
    return REDIS_OK; /* unreached */
}

接下来我们看一下子进程是如何完成该工作的：

/* Write a sequence of commands able to fully rebuild the dataset into
 * "filename". Used both by REWRITEAOF and BGREWRITEAOF.
 *
 * 将一集足以还原当前数据集的命令写入到 filename 指定的文件中。
 *
 * 这个函数被 REWRITEAOF 和 BGREWRITEAOF 两个命令调用。
 * （REWRITEAOF 似乎已经是一个废弃的命令）
 *
 * In order to minimize the number of commands needed in the rewritten
 * log Redis uses variadic commands when possible, such as RPUSH, SADD
 * and ZADD. However at max REDIS_AOF_REWRITE_ITEMS_PER_CMD items per time
 * are inserted using a single command. 
 *
 * 为了最小化重建数据集所需执行的命令数量，
 * Redis 会尽可能地使用接受可变参数数量的命令，比如 RPUSH 、SADD 和 ZADD 等。
 * 不过单个命令每次处理的元素数量不能超过 REDIS_AOF_REWRITE_ITEMS_PER_CMD 。
 */
int rewriteAppendOnlyFile(char *filename) {
    dictIterator *di = NULL;
    dictEntry *de;
    rio aof;
    FILE *fp;
    char tmpfile[256];
    int j;
    long long now = mstime();

    /* Note that we have to use a different temp name here compared to the
     * one used by rewriteAppendOnlyFileBackground() function. 
     *
     * 创建临时文件
     *
     * 注意这里创建的文件名和 rewriteAppendOnlyFileBackground() 创建的文件名稍有不同
     * 一个是temp-rewriteaof-bg-%d.aof
     * 另一个是temp-rewriteaof-%d.aof
     */
    snprintf(tmpfile,256,"temp-rewriteaof-%d.aof", (int) getpid());
    fp = fopen(tmpfile,"w");
    if (!fp) {
        redisLog(REDIS_WARNING, "Opening the temp file for AOF rewrite in rewriteAppendOnlyFile(): %s", strerror(errno));
        return REDIS_ERR;
    }

    // 初始化文件 io
    rioInitWithFile(&aof,fp);

    // 设置每写入 REDIS_AOF_AUTOSYNC_BYTES 字节
    // 就执行一次 FSYNC(fsync函数同步内存中所有已修改的文件数据到储存设备。参数fd是该进程打开来的文件描述符。 函数成功执行时，返回0。失败返回-1)
    // 防止缓存中积累太多命令内容，造成 I/O 阻塞时间过长
    if (server.aof_rewrite_incremental_fsync)
        rioSetAutoSync(&aof,REDIS_AOF_AUTOSYNC_BYTES);

    // 遍历所有数据库
    for (j = 0; j < server.dbnum; j++) {

        char selectcmd[] = "*2
$6
SELECT
";

        redisDb *db = server.db+j;

        // 指向键空间
        dict *d = db->dict;
        if (dictSize(d) == 0) continue;

        // 创建键空间迭代器
        di = dictGetSafeIterator(d);
        if (!di) {
            fclose(fp);
            return REDIS_ERR;
        }

        /* SELECT the new DB 
         *
         * 首先写入 SELECT 命令，确保之后的数据会被插入到正确的数据库上
         * (这一点可以自行打开appendonly.aof查看相应的select语句的保存)
         */
        if (rioWrite(&aof,selectcmd,sizeof(selectcmd)-1) == 0) goto werr;
        if (rioWriteBulkLongLong(&aof,j) == 0) goto werr;

        /* Iterate this DB writing every entry 
         *
         * 遍历数据库所有键，并通过命令将它们的当前状态（值）记录到新 AOF 文件中
         */
        while((de = dictNext(di)) != NULL) {
            sds keystr;
            robj key, *o;
            long long expiretime;

            // 取出键
            keystr = dictGetKey(de);

            // 取出值
            o = dictGetVal(de);
            initStaticStringObject(key,keystr);

            // 取出过期时间
            expiretime = getExpire(db,&key);

            /* If this key is already expired skip it 
             *
             * 如果键已经过期，那么跳过它，不保存
             */
            if (expiretime != -1 && expiretime < now) continue;

            /* Save the key and associated value 
             *
             * 根据值的类型，选择适当的命令来保存值
             */
            if (o->type == REDIS_STRING) {
                /* Emit a SET command */
                char cmd[]="*3
$3
SET
";
                if (rioWrite(&aof,cmd,sizeof(cmd)-1) == 0) goto werr;
                /* Key and value */
                if (rioWriteBulkObject(&aof,&key) == 0) goto werr;
                if (rioWriteBulkObject(&aof,o) == 0) goto werr;
            } else if (o->type == REDIS_LIST) {
                if (rewriteListObject(&aof,&key,o) == 0) goto werr;
            } else if (o->type == REDIS_SET) {
                if (rewriteSetObject(&aof,&key,o) == 0) goto werr;
            } else if (o->type == REDIS_ZSET) {
                if (rewriteSortedSetObject(&aof,&key,o) == 0) goto werr;
            } else if (o->type == REDIS_HASH) {
                if (rewriteHashObject(&aof,&key,o) == 0) goto werr;
            } else {
                redisPanic("Unknown object type");
            }

            /* Save the expire time 
             *
             * 保存键的过期时间
             */
            if (expiretime != -1) {
                char cmd[]="*3
$9
PEXPIREAT
";

                // 写入 PEXPIREAT expiretime 命令
                if (rioWrite(&aof,cmd,sizeof(cmd)-1) == 0) goto werr;
                if (rioWriteBulkObject(&aof,&key) == 0) goto werr;
                if (rioWriteBulkLongLong(&aof,expiretime) == 0) goto werr;
            }
        }

        // 释放迭代器
        dictReleaseIterator(di);
    }

    /* Make sure data will not remain on the OS's output buffers */
    // 冲洗并关闭新 AOF 文件(写入磁盘)
    if (fflush(fp) == EOF) goto werr;
    if (aof_fsync(fileno(fp)) == -1) goto werr;
    if (fclose(fp) == EOF) goto werr;

    /* Use RENAME to make sure the DB file is changed atomically only
     * if the generate DB file is ok. 
     *
     * 原子地改名，用重写后的新 AOF 文件覆盖旧 AOF 文件
     */
    if (rename(tmpfile,filename) == -1) {
        redisLog(REDIS_WARNING,"Error moving temp append only file on the final destination: %s", strerror(errno));
        unlink(tmpfile);
        return REDIS_ERR;
    }

    redisLog(REDIS_NOTICE,"SYNC append only file rewrite performed");

    return REDIS_OK;

werr:
    fclose(fp);
    unlink(tmpfile);
    redisLog(REDIS_WARNING,"Write error writing append only file on disk: %s", strerror(errno));
    if (di) dictReleaseIterator(di);
    return REDIS_ERR;
}

至此子进程完成rewrite操作。那么父进程也就是主线程是在什么时候获得子进程退出状态，并且做了些什么操作？

在上面的serverCron中可以看到：

// 接收子进程发来的信号，非阻塞
        if ((pid = wait3(&statloc,WNOHANG,NULL)) != 0) {
            int exitcode = WEXITSTATUS(statloc);
            int bysignal = 0;
            
            if (WIFSIGNALED(statloc)) bysignal = WTERMSIG(statloc);

            // BGSAVE 执行完毕
            if (pid == server.rdb_child_pid) {
                backgroundSaveDoneHandler(exitcode,bysignal);

            // BGREWRITEAOF 执行完毕
            } else if (pid == server.aof_child_pid) {
                backgroundRewriteDoneHandler(exitcode,bysignal);

            } else {
                redisLog(REDIS_WARNING,
                    "Warning, detected child with unmatched pid: %ld",
                    (long)pid);
            }
            updateDictResizePolicy();

 即父进程在serverCron里通过server.bgrewritechildpid来判断是否需要等待子进程退出的信号。

进一步我们来看一下backgroundRewriteDoneHandler作了哪些操作：（注意这里是AOF的难点，使用了很强的技巧，反正我是看了好半天，才略懂）

/* A background append only file rewriting (BGREWRITEAOF) terminated its work.
 * Handle this. 
 *
 * 当子线程完成 AOF 重写时，父进程调用这个函数。
 */
void backgroundRewriteDoneHandler(int exitcode, int bysignal) {
    if (!bysignal && exitcode == 0) {
        int newfd, oldfd;
        char tmpfile[256];
        long long now = ustime();

        redisLog(REDIS_NOTICE,
            "Background AOF rewrite terminated with success");

        /* Flush the differences accumulated by the parent to the
         * rewritten AOF. */
        // 打开保存新 AOF 文件内容的临时文件
        snprintf(tmpfile,256,"temp-rewriteaof-bg-%d.aof",
            (int)server.aof_child_pid);
        newfd = open(tmpfile,O_WRONLY|O_APPEND);
        if (newfd == -1) {
            redisLog(REDIS_WARNING,
                "Unable to open the temporary AOF produced by the child: %s", strerror(errno));
            goto cleanup;
        }

        // 将累积的重写缓存写入到临时文件中
        // 这个函数调用的 write 操作会阻塞主进程
        if (aofRewriteBufferWrite(newfd) == -1) {
            redisLog(REDIS_WARNING,
                "Error trying to flush the parent diff to the rewritten AOF: %s", strerror(errno));
            close(newfd);
            goto cleanup;
        }

        redisLog(REDIS_NOTICE,
            "Parent diff successfully flushed to the rewritten AOF (%lu bytes)", aofRewriteBufferSize());

        /* The only remaining thing to do is to rename the temporary file to
         * the configured file and switch the file descriptor used to do AOF
         * writes. We don't want close(2) or rename(2) calls to block the
         * server on old file deletion.
         *
         * 剩下的工作就是将临时文件改名为 AOF 程序指定的文件名，
         * 并将新文件的 fd 设为 AOF 程序的写目标。
         *
         * 不过这里有一个问题 ——
         * 我们不想 close(2) 或者 rename(2) 在删除旧文件时阻塞。
         *
         * There are two possible scenarios:
         *
         * 以下是两个可能的场景：
         *
         * 1) AOF is DISABLED and this was a one time rewrite. The temporary
         * file will be renamed to the configured file. When this file already
         * exists, it will be unlinked, which may block the server.
         *
         * AOF 被关闭，这个是一次单次的写操作。
         * 临时文件会被改名为 AOF 文件。
         * 本来已经存在的 AOF 文件会被 unlink ，这可能会阻塞服务器。
         *
         * 2) AOF is ENABLED and the rewritten AOF will immediately start
         * receiving writes. After the temporary file is renamed to the
         * configured file, the original AOF file descriptor will be closed.
         * Since this will be the last reference to that file, closing it
         * causes the underlying file to be unlinked, which may block the
         * server.
         *
         * AOF 被开启，并且重写后的 AOF 文件会立即被用于接收新的写入命令。
         * 当临时文件被改名为 AOF 文件时，原来的 AOF 文件描述符会被关闭。
         * 因为 Redis 会是最后一个引用这个文件的进程，
         * 所以关闭这个文件会引起 unlink ，这可能会阻塞服务器。
         *
         * To mitigate the blocking effect of the unlink operation (either
         * caused by rename(2) in scenario 1, or by close(2) in scenario 2), we
         * use a background thread to take care of this. First, we
         * make scenario 1 identical to scenario 2 by opening the target file
         * when it exists. The unlink operation after the rename(2) will then
         * be executed upon calling close(2) for its descriptor. Everything to
         * guarantee atomicity for this switch has already happened by then, so
         * we don't care what the outcome or duration of that close operation
         * is, as long as the file descriptor is released again. 
         *
         * 为了避免出现阻塞现象，程序会将 close(2) 放到后台线程执行，
         * 这样服务器就可以持续处理请求，不会被中断。
         */
        if (server.aof_fd == -1) {
            /* AOF disabled */

             /* Don't care if this fails: oldfd will be -1 and we handle that.
              * One notable case of -1 return is if the old file does
              * not exist. */
             oldfd = open(server.aof_filename,O_RDONLY|O_NONBLOCK);
        } else {
            /* AOF enabled */
            oldfd = -1; /* We'll set this to the current AOF filedes later. */
        }

        /* Rename the temporary file. This will not unlink the target file if
         * it exists, because we reference it with "oldfd". 
         *
         * 对临时文件进行改名，替换现有的 AOF 文件。
         *
         * 旧的 AOF 文件不会在这里被 unlink ，因为 oldfd 引用了它。
         */
        if (rename(tmpfile,server.aof_filename) == -1) {
            redisLog(REDIS_WARNING,
                "Error trying to rename the temporary AOF file: %s", strerror(errno));
            close(newfd);
            if (oldfd != -1) close(oldfd);
            goto cleanup;
        }

        if (server.aof_fd == -1) {
            /* AOF disabled, we don't need to set the AOF file descriptor
             * to this new file, so we can close it. 
             *
             * AOF 被关闭，直接关闭 AOF 文件，
             * 因为关闭 AOF 本来就会引起阻塞，所以这里就算 close 被阻塞也无所谓
             */
            close(newfd);
        } else {
            /* AOF enabled, replace the old fd with the new one. 
             *
             * 用新 AOF 文件的 fd 替换原来 AOF 文件的 fd
             */
            oldfd = server.aof_fd;
            server.aof_fd = newfd;

            // 因为前面进行了 AOF 重写缓存追加，所以这里立即 fsync 一次
            if (server.aof_fsync == AOF_FSYNC_ALWAYS)
                aof_fsync(newfd);
            else if (server.aof_fsync == AOF_FSYNC_EVERYSEC)
                aof_background_fsync(newfd);

            // 强制引发 SELECT
            server.aof_selected_db = -1; /* Make sure SELECT is re-issued */

            // 更新 AOF 文件的大小
            aofUpdateCurrentSize();

            // 记录前一次重写时的大小
            server.aof_rewrite_base_size = server.aof_current_size;

            /* Clear regular AOF buffer since its contents was just written to
             * the new AOF from the background rewrite buffer. 
             *
             * 清空 AOF 缓存，因为它的内容已经被写入过了，没用了
             */
            sdsfree(server.aof_buf);
            server.aof_buf = sdsempty();
        }

        server.aof_lastbgrewrite_status = REDIS_OK;

        redisLog(REDIS_NOTICE, "Background AOF rewrite finished successfully");

        /* Change state from WAIT_REWRITE to ON if needed 
         *
         * 如果是第一次创建 AOF 文件，那么更新 AOF 状态
         * 把close old-aof-file的工作交给backgroud thread来执行
         */
        if (server.aof_state == REDIS_AOF_WAIT_REWRITE)
            server.aof_state = REDIS_AOF_ON;

        /* Asynchronously close the overwritten AOF. 
         *
         * 异步关闭旧 AOF 文件
         * 把close old-aof-file的工作交给backgroud thread来执行
         */
        if (oldfd != -1) bioCreateBackgroundJob(REDIS_BIO_CLOSE_FILE,(void*)(long)oldfd,NULL,NULL);

        redisLog(REDIS_VERBOSE,
            "Background AOF rewrite signal handler took %lldus", ustime()-now);

    // BGREWRITEAOF 重写出错
    } else if (!bysignal && exitcode != 0) {
        server.aof_lastbgrewrite_status = REDIS_ERR;

        redisLog(REDIS_WARNING,
            "Background AOF rewrite terminated with error");

    // 未知错误
    } else {
        server.aof_lastbgrewrite_status = REDIS_ERR;

        redisLog(REDIS_WARNING,
            "Background AOF rewrite terminated by signal %d", bysignal);
    }

cleanup:

    // 清空 AOF 缓冲区
    aofRewriteBufferReset();

    // 移除临时文件
    aofRemoveTempFile(server.aof_child_pid);

    // 重置默认属性
    server.aof_child_pid = -1;
    server.aof_rewrite_time_last = time(NULL)-server.aof_rewrite_time_start;
    server.aof_rewrite_time_start = -1;

    /* Schedule a new rewrite if we are waiting for it to switch the AOF ON. */
    if (server.aof_state == REDIS_AOF_WAIT_REWRITE)
        server.aof_rewrite_scheduled = 1;
}

 关于backgroundRewriteDoneHandler其中为什么这么做，可以参考文章：http://www.hoterran.info/redis-aof-backgroud-thread。