🚄 前提概要

随着redis的运行，AOF会不断膨胀（对于一个key会有多条AOF日志），导致通过aof恢复数据时，耗费大量不必要的时间。redis提供的解决方案是AOF Rewrite。

根据DB的内容，对于每个key，生成一条日志，AOF触发的时机。

• 1）用户调用bgrewriteaof命令

• 2）AOF日志大小超过预设的配置的阈值。

🚄 AOF Rewrite触发时机

首先看一下，bgrewriteaof的处理函数：

void bgrewriteaofCommand(redisClient *c) {
    if (server.aof_child_pid != -1) {
        addReplyError(c,"Background append only file rewriting already in progress");
    } else if (server.rdb_child_pid != -1) {
        server.aof_rewrite_scheduled = 1;
        addReplyStatus(c,"Background append only file rewriting scheduled");
    } else if (rewriteAppendOnlyFileBackground() == REDIS_OK) {
        addReplyStatus(c,"Background append only file rewriting started");
    } else {
        addReply(c,shared.err);
    }
}

• aof_child_pid：是指的进行aof rewrite进程的 pid（都是后台进程）

• rdb_child_pid：是指的进行rdb dump的进程pid。（都是后台进程）

下面，看一下serverCron中是如何触发aof rewrite的。

第一个触发点是，避免与rdb dump冲突，延迟触发rewrite。

/* Start a scheduled AOF rewrite if this was requested by the user while
 * a BGSAVE was in progress. */
if (server.rdb_child_pid == -1 && server.aof_child_pid == -1 &&
    server.aof_rewrite_scheduled){
    rewriteAppendOnlyFileBackground();
}

需要确认当前没有aof rewrite和rdb dump在进行（-1），并且设置了aof_rewrite_scheduled，调用rewirteAppendOnlyFileBackground进行aof rewrite。

第二个触发位置是aof文件的大小超过预定的百分比。

/* Trigger an AOF rewrite if needed */
if (server.rdb_child_pid == -1 &&
    server.aof_child_pid == -1 &&
    server.aof_rewrite_perc &&
    server.aof_current_size > server.aof_rewrite_min_size){
    long long base = server.aof_rewrite_base_size ?
                        server.aof_rewrite_base_size : 1;
    long long growth = (server.aof_current_size*100/base) - 100;
    if (growth >= server.aof_rewrite_perc) {
       redisLog(REDIS_NOTICE,"Starting automatic rewriting of AOF on %lld%% growth",growth);
       rewriteAppendOnlyFileBackground();
    }
}

当aof文件超过了预定的最小值，并且超过了上一次aof文件的一定百分比，则会触发aof rewrite。

🚄 AOF Rewrite 核心流程

rewrite的大致流程是：

下面看具体代码，首先是rewriteAppendOnlyFileBackground。

pid_t childpid;
long long start;

// 
// 避免同时多个进程进行rewrite
// 
if (server.aof_child_pid != -1) 
  return REDIS_ERR;

如果有其他 aof rewrite进程正在进行，直接返回错误。

start = ustime();
if ((childpid = fork()) == 0) {
  char tmpfile[256];
	/* Child */
    // 
    // 子进程不能接受连接
    // 
    closeListeningSockets(0);
    redisSetProcTitle("redis-aof-rewrite");
    // 
    // 生成临时aof文件名
    // 
    snprintf(tmpfile,256,"temp-rewriteaof-bg-%d.aof", (int) getpid());
    if (rewriteAppendOnlyFile(tmpfile) == REDIS_OK) {
        size_t private_dirty = zmalloc_get_private_dirty();
        if (private_dirty) {
            redisLog(REDIS_NOTICE,
                "AOF rewrite: %zu MB of memory used by copy-on-write",
                private_dirty/(1024*1024));
        }
        exitFromChild(0);
    } else {
        exitFromChild(1);
}

下面看一下父进程的流程：

} else {
    /* Parent */
    server.stat_fork_time = ustime()-start;
    server.stat_fork_rate = (double) zmalloc_used_memory() * 1000000 / server.stat_fork_time / (1024*1024*1024); /* GB per second. */
    latencyAddSampleIfNeeded("fork",server.stat_fork_time/1000);
    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);
    server.aof_rewrite_scheduled = 0;
    server.aof_rewrite_time_start = time(NULL);
    server.aof_child_pid = childpid;
    updateDictResizePolicy();
    /* We set appendseldb 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. */
    server.aof_selected_db = -1;
    replicationScriptCacheFlush();
    return REDIS_OK;
}

父进程首先统计fork耗时并采样。

• 如果fork失败，记录日志并返回错误。

• 如果fork成功，对aof_rewrite_scheduled清零，记录rewrite开始时间以及aof_child_pid（redis通过这个属性判断是否有aof rewrite在进行）。

调用updateDictResizePolicy调整db的key space的rehash策略，由于创建了子进程，避免copy-on-write复制大量内存页，这里会禁止dict的rehash。

将aof_selected_db置为-1，目的是，下一条aof会首先生成一条select db的日志，同时会写到aof_rewrite_buf中，这样就可以将aof_rewrite_buf正常的追加到rewrite之后的文件。replicationScriptCacheFlush暂时没看到这。

下面看一下子进程进行aof rewrite的过程，进入rewriteAppendOnlyFile函数。大体上，就是遍历所有key，进行序列化，然后记录到aof文件中。

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. */

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;
}

获取当前时间，生成临时文件名并创建该文件。

rioInitWithFile(&aof,fp);
if (server.aof_rewrite_incremental_fsync)
   rioSetAutoSync(&aof,REDIS_AOF_AUTOSYNC_BYTES);

rio就是面向流的I/O接口，底层可以有不同实现，目前提供了文件和内存buffer的实现。

这里对rio进行初始化。如果配置了server.aof_rewrite_incremental_fsync，则在写aof时会增量地进行fsync，这里配置的是每写入32M就sync一次。避免集中sync导致磁盘跑满。接下来是一个循环，用于遍历redis的每个db，对其进行rewirte。直接看循环内部：

    char selectcmd[] = "*2\r\n$6\r\nSELECT\r\n";
    redisDb *db = server.db+j;
    dict *d = db->dict;
    if (dictSize(d) == 0) continue;
    di = dictGetSafeIterator(d);
    if (!di) {
        fclose(fp);
        return REDIS_ERR;
    }
    /* SELECT the new DB */
    if (rioWrite(&aof,selectcmd,sizeof(selectcmd)-1) == 0) goto werr;
    if (rioWriteBulkLongLong(&aof,j) == 0) goto werr;

首先，生成对应db的select命令，然后查看如果db为空的话，就跳过，rewrite下一个db。然后获取该db的迭代器，如果获取失败，直接返回错误。最后将select db的命令写入文件。接下来还是一个循环，用于遍历db的每一个key，生成相应的命令。

while ((de = dictNext(di)) != NULL) {
    // ...
}
dictReleaseIterator(di);

继续看循环内部：

        sds keystr;
        robj key, *o;
        long long expiretime;

        keystr = dictGetKey(de);
        o = dictGetVal(de);
        initStaticStringObject(key,keystr);

        expiretime = getExpire(db,&key);

        /* If this key is already expired skip it */
        if (expiretime != -1 && expiretime < now) continue;

de是dict的一个entry，包含了key和value。这里，首先获取key和value，并将key转换成robj类型。然后，获取key对应的超时时间。如果已经超时，则跳过这个key。

            /* Save the key and associated value */
            if (o->type == REDIS_STRING) {
                /* Emit a SET command */
                char cmd[]="*3\r\n$3\r\nSET\r\n";
                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");
            }

接下来，根据对象的类型，序列化成相应的命令。并将命令写入aof文件中。具体各个对象的序列化，这里不再详述。

/* Make sure data will not remain on the OS's output buffers */
if (fflush(fp) == EOF) goto werr;
if (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. */
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;

调用fflush，fsync将数据落地到磁盘，最后close文件。将临时文件重命名，确保生成的aof文件完全ok，避免出现aof不完整的情况。最后，打印日志并返回。

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;

在打开文件后，任何一个步出错，都会跳到werr，进行错误处理。这里，需要将文件close，删除临时文件，如果dict的迭代器没有释放的话，需要进行释放。最后，返回error。

到这，子进程的aof rewrite任务就完成了，现在rewrite后的文件已经生成，但是在rewrite过程中得日志并没有记录到aof文件，所以还需部分收尾工作，这是在主进程中完成的。

🚄AOF Rewrite Buffer追加

多进程编程中，子进程退出后，父进程需要对其进行清理，否则子进程会编程僵尸进程。同样是在serverCron函数中，主进程完成对rewrite进程的清理。

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

        /* Check if a background saving or AOF rewrite in progress terminated. */
    if (server.rdb_child_pid != -1 || server.aof_child_pid != -1) {
        int statloc;
        pid_t pid;
 
        if ((pid = wait3(&statloc,WNOHANG,NULL)) != 0) {
            int exitcode = WEXITSTATUS(statloc);
            int bysignal = 0;
 
            if (WIFSIGNALED(statloc)) bysignal = WTERMSIG(statloc);
 
            if (pid == server.rdb_child_pid) {
                backgroundSaveDoneHandler(exitcode,bysignal);
            } 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 {

如果正在进程rdb dump或者aof rewrite，主进程会非阻塞的调用wait3函数，以便在子进程退出后，获取其退出状态。如果退出的进程是aof rewrite进程的话，会调用backgroundRewriteDoneHandler函数进行最后的收尾工作。下面看一下这个函数。

如果正常退出的情况下，就是没有被信号kill，并且退出码等于0。

        int newfd, oldfd;
        char tmpfile[256];
        long long now = ustime();
        mstime_t latency;
 
        redisLog(REDIS_NOTICE,
            "Background AOF rewrite terminated with success");
 
        /* Flush the differences accumulated by the parent to the
         * rewritten AOF. */
        latencyStartMonitor(latency);
        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;
        }

首先是记录日志，然后打开临时写入的rewrite文件。

    // 
    // 将rewrite buf追加到文件
    // 
    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;
    }
    latencyEndMonitor(latency);
    latencyAddSampleIfNeeded("aof-rewrite-diff-write",latency);

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

接下来，将aof rewrite buffer追加到文件。

        /* 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.
         *
         * There are two possible scenarios:
         *
         * 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.
         *
         * 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.
         *
         * 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. */
        if (server.aof_fd == -1) {
            // 
            // 没有开启AOF，由命令触发的aof rewrite
            // 
            /* 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". */
        latencyStartMonitor(latency);
        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;
        }
        latencyEndMonitor(latency);
        latencyAddSampleIfNeeded("aof-rename",latency);
 
        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. */
            close(newfd);
        } else {
            /* AOF enabled, replace the old fd with the new one. */
            oldfd = server.aof_fd;
            server.aof_fd = newfd;
            if (server.aof_fsync == AOF_FSYNC_ALWAYS)
                aof_fsync(newfd);
            else if (server.aof_fsync == AOF_FSYNC_EVERYSEC)
                aof_background_fsync(newfd);
            server.aof_selected_db = -1; /* Make sure SELECT is re-issued */
            aofUpdateCurrentSize();
            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. */
            sdsfree(server.aof_buf);
            server.aof_buf = sdsempty();
        }

然后，将临时文件重命名为最终的aof文件。

        server.aof_lastbgrewrite_status = REDIS_OK;
 
        redisLog(REDIS_NOTICE, "Background AOF rewrite finished successfully");
        /* Change state from WAIT_REWRITE to ON if needed */
        if (server.aof_state == REDIS_AOF_WAIT_REWRITE)
            server.aof_state = REDIS_AOF_ON;
 
        /* Asynchronously close the overwritten AOF. */
        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);

最后，更新状态，异步关闭之前的aof文件。如果rewrite子进程异常退出，由信号kill或者退出码非0，则只是记录 日志。

} else if (!bysignal && exitcode != 0) {
    server.aof_lastbgrewrite_status = REDIS_ERR;

    redisLog(REDIS_WARNING,
        "Background AOF rewrite terminated with error");
} else {
    server.aof_lastbgrewrite_status = REDIS_ERR;

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

在追加rewrite buffer或者重命名文件失败时，需要进行清理工作，有cleanup分支处理：

cleanup:
    aofRewriteBufferReset();
    aofRemoveTempFile(server.aof_child_pid);
    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;