Redis数据持久化机制AOF原理分析二

本文所引用的源码全部来自Redis2.8.2版本。 Redis AOF数据持久化机制的实现相关代码是redis.c, redis.h, aof.c, bio.c, rio.c, config.c 在阅读本文之前请先阅读Redis数据持久化机制AOF原理分析之配置详解文章，了解AOF相关参数的解析，文章链接 http://blog

本文所引用的源码全部来自Redis2.8.2版本。

Redis AOF数据持久化机制的实现相关代码是redis.c, redis.h, aof.c, bio.c, rio.c, config.c

在阅读本文之前请先阅读Redis数据持久化机制AOF原理分析之配置详解文章，了解AOF相关参数的解析，文章链接

http://blog.csdn.net/acceptedxukai/article/details/18135219

接着上一篇文章，本文将介绍Redis是如何实现AOF rewrite的。

转载请注明，文章出自http://blog.csdn.net/acceptedxukai/article/details/18181563

AOF rewrite的触发机制

如果Redis只是将客户端修改数据库的指令重现存储在AOF文件中，那么AOF文件的大小会不断的增加，因为AOF文件只是简单的重现存储了客户端的指令，而并没有进行合并。对于该问题最简单的处理方式，即当AOF文件满足一定条件时就对AOF进行rewrite，rewrite是根据当前内存数据库中的数据进行遍历写到一个临时的AOF文件，待写完后替换掉原来的AOF文件即可。

Redis触发AOF rewrite机制有三种：

1、Redis Server接收到客户端发送的BGREWRITEAOF指令请求，如果当前AOF/RDB数据持久化没有在执行，那么执行，反之，等当前AOF/RDB数据持久化结束后执行AOF rewrite

2、在Redis配置文件redis.conf中，用户设置了auto-aof-rewrite-percentage和auto-aof-rewrite-min-size参数，并且当前AOF文件大小server.aof_current_size大于auto-aof-rewrite-min-size(server.aof_rewrite_min_size)，同时AOF文件大小的增长率大于auto-aof-rewrite-percentage(server.aof_rewrite_perc)时，会自动触发AOF rewrite

3、用户设置“config set appendonly yes”开启AOF的时，调用startAppendOnly函数会触发rewrite

下面分别介绍上述三种机制的处理.

接收到BGREWRITEAOF指令

void bgrewriteaofCommand(redisClient *c) {
    //AOF rewrite正在执行，那么直接返回
    if (server.aof_child_pid != -1) {
        addReplyError(c,"Background append only file rewriting already in progress");
    } else if (server.rdb_child_pid != -1) {
        //AOF rewrite未执行，但RDB数据持久化正在执行，那么设置AOF rewrite状态为scheduled
        //待RDB结束后执行AOF rewrite
        server.aof_rewrite_scheduled = 1;
        addReplyStatus(c,"Background append only file rewriting scheduled");
    } else if (rewriteAppendOnlyFileBackground() == REDIS_OK) {
        //直接执行AOF rewrite
        addReplyStatus(c,"Background append only file rewriting started");
    } else {
        addReply(c,shared.err);
    }
}

当AOF rewrite请求被挂起时，在serverCron函数中，会处理。

/* Start a scheduled AOF rewrite if this was requested by the user while
     * a BGSAVE was in progress. */
    // 如果用户执行 BGREWRITEAOF 命令的话，在后台开始 AOF 重写
    //当用户执行BGREWRITEAOF命令时，如果RDB文件正在写，那么将server.aof_rewrite_scheduled标记为1
    //当RDB文件写完后开启AOF rewrite
    if (server.rdb_child_pid == -1 && server.aof_child_pid == -1 &&
        server.aof_rewrite_scheduled)
    {
        rewriteAppendOnlyFileBackground();
    }

Server自动对AOF进行rewrite

在serverCron函数中会周期性判断

/* Trigger an AOF rewrite if needed */
         //满足一定条件rewrite AOF文件
         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();
            }
         }

config set appendonly yes

当客户端发送该指令时，config.c中的configSetCommand函数会做出响应，startAppendOnly函数会执行AOF rewrite

if (!strcasecmp(c->argv[2]->ptr,"appendonly")) {
	int enable = yesnotoi(o->ptr);

	if (enable == -1) goto badfmt;
	if (enable == 0 && server.aof_state != REDIS_AOF_OFF) {//appendonly no 关闭AOF
		stopAppendOnly();
	} else if (enable && server.aof_state == REDIS_AOF_OFF) {//appendonly yes rewrite AOF
		if (startAppendOnly() == REDIS_ERR) {
			addReplyError(c,
				"Unable to turn on AOF. Check server logs.");
			return;
		}
	}
}

int startAppendOnly(void) {
    server.aof_last_fsync = server.unixtime;
    server.aof_fd = open(server.aof_filename,O_WRONLY|O_APPEND|O_CREAT,0644);
    redisAssert(server.aof_state == REDIS_AOF_OFF);
    if (server.aof_fd == -1) {
        redisLog(REDIS_WARNING,"Redis needs to enable the AOF but can't open the append only file: %s",strerror(errno));
        return REDIS_ERR;
    }
    if (rewriteAppendOnlyFileBackground() == REDIS_ERR) {//rewrite
        close(server.aof_fd);
        redisLog(REDIS_WARNING,"Redis needs to enable the AOF but can't trigger a background AOF rewrite operation. Check the above logs for more info about the error.");
        return REDIS_ERR;
    }
    /* We correctly switched on AOF, now wait for the rerwite to be complete
     * in order to append data on disk. */
    server.aof_state = REDIS_AOF_WAIT_REWRITE;
    return REDIS_OK;
}

Redis AOF rewrite机制的实现

从上述分析可以看出rewrite的实现全部依靠rewriteAppendOnlyFileBackground函数，下面分析该函数，通过下面的代码可以看出，Redis是fork出一个子进程来操作AOF rewrite，然后子进程调用rewriteAppendOnlyFile函数，将数据写到一个临时文件temp-rewriteaof-bg-%d.aof中。如果子进程完成会通过exit(0)函数通知父进程rewrite结束，在serverCron函数中使用wait3函数接收子进程退出状态，然后执行后续的AOF rewrite的收尾工作，后面将会分析。父进程的工作主要包括清楚server.aof_rewrite_scheduled标志，记录子进程IDserver.aof_child_pid = childpid，记录rewrite的开始时间server.aof_rewrite_time_start = time(NULL)等。

int rewriteAppendOnlyFileBackground(void) {
    pid_t childpid;
    long long start;

    // 后台重写正在执行
    if (server.aof_child_pid != -1) return REDIS_ERR;
    start = ustime();
    if ((childpid = fork()) == 0) {
        char tmpfile[256];

        /* Child */
        closeListeningSockets(0);//
        redisSetProcTitle("redis-aof-rewrite");
        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);
        }
    } else {
        /* Parent */
        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);
        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;
    }
    return REDIS_OK; /* unreached */
}

接下来介绍rewriteAppendOnlyFile函数，该函数的主要工作为：遍历所有数据库中的数据，将其写入到临时文件temp-rewriteaof-%d.aof中，写入函数定义在rio.c中，比较简单，然后将数据刷新到硬盘中，然后将文件名rename为其调用者给定的临时文件名，注意仔细看代码，这里并没有修改为正式的AOF文件名。在写入文件时如果设置server.aof_rewrite_incremental_fsync参数，那么在rioWrite函数中fwrite部分数据就会将数据fsync到硬盘中，来保证数据的正确性。

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. */
    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;
    }

    rioInitWithFile(&aof,fp); //初始化读写函数，rio.c
    //设置r->io.file.autosync = bytes;每32M刷新一次
    if (server.aof_rewrite_incremental_fsync)
        rioSetAutoSync(&aof,REDIS_AOF_AUTOSYNC_BYTES);
    for (j = 0; j 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;

        /* Iterate this DB writing every entry */
        while((de = dictNext(di)) != NULL) {
            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 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");
            }
            /* Save the expire time */
            if (expiretime != -1) {
                char cmd[]="*3\r\n$9\r\nPEXPIREAT\r\n";
                if (rioWrite(&aof,cmd,sizeof(cmd)-1) == 0) goto werr;
                if (rioWriteBulkObject(&aof,&key) == 0) goto werr;
                if (rioWriteBulkLongLong(&aof,expiretime) == 0) goto werr;
            }
        }
        dictReleaseIterator(di);
    }

    /* Make sure data will not remain on the OS's output buffers */
    fflush(fp);
    aof_fsync(fileno(fp));//将tempfile文件刷新到硬盘
    fclose(fp);

    /* Use RENAME to make sure the DB file is changed atomically only
     * if the generate DB file is ok. */
    if (rename(tmpfile,filename) == -1) {//重命名文件名，注意rename后的文件也是一个临时文件
        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;
}

AOF rewrite工作到这里已经结束一半，上一篇文章提到如果server.aof_state != REDIS_AOF_OFF，那么就会将客户端请求指令修改的数据通过feedAppendOnlyFile函数追加到AOF文件中，那么此时AOF已经rewrite了，必须要处理此时出现的差异数据，记得在feedAppendOnlyFile函数中有这么一段代码

if (server.aof_child_pid != -1)
        aofRewriteBufferAppend((unsigned char*)buf,sdslen(buf));

如果AOF rewrite正在进行，那么就将修改数据的指令字符串存储到server.aof_rewrite_buf_blocks链表中，等待AOF rewrite子进程结束后处理，处理此部分数据的代码在serverCron函数中。需要指出的是wait3函数我不了解，可能下面注释会有点问题。

/* Check if a background saving or AOF rewrite in progress terminated. */
//如果RDB bgsave或AOF rewrite子进程已经执行,通过获取子进程的退出状态，对后续的工作进行处理
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);//获取退出的状态
		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);
		}
		// 如果 BGSAVE 和 BGREWRITEAOF 都已经完成，那么重新开始 REHASH
		updateDictResizePolicy();
	}
}

对于AOF rewrite期间出现的差异数据，Server通过backgroundSaveDoneHandler函数将server.aof_rewrite_buf_blocks链表中数据追加到新的AOF文件中。 backgroundSaveDoneHandler函数执行步骤：
1、通过判断子进程的退出状态，正确的退出状态为exit(0)，即exitcode为0，bysignal我不清楚具体意义，如果退出状态正确，backgroundSaveDoneHandler函数才会开始处理 2、通过对rewriteAppendOnlyFileBackground函数的分析，可以知道rewrite后的AOF临时文件名为temp-rewriteaof-bg-%d.aof(%d=server.aof_child_pid)中，接着需要打开此临时文件 3、调用aofRewriteBufferWrite函数将server.aof_rewrite_buf_blocks中差异数据写到该临时文件中 4、如果旧的AOF文件未打开，那么打开旧的AOF文件，将文件描述符赋值给临时变量oldfd 5、将临时的AOF文件名rename为正常的AOF文件名 6、如果旧的AOF文件未打开，那么此时只需要关闭新的AOF文件，此时的server.aof_rewrite_buf_blocks数据应该为空；如果旧的AOF是打开的，那么将server.aof_fd指向newfd,然后根据相应的fsync策略将数据刷新到硬盘上 7、调用aofUpdateCurrentSize函数统计AOF文件的大小，更新server.aof_rewrite_base_size，为serverCron中自动AOF rewrite做相应判断 8、如果之前是REDIS_AOF_WAIT_REWRITE状态，则设置server.aof_state为REDIS_AOF_ON，因为只有“config set appendonly yes”指令才会设置这个状态，也就是需要写完快照后，立即打开AOF；而BGREWRITEAOF不需要打开AOF 9、调用后台线程去关闭旧的AOF文件下面是backgroundSaveDoneHandler函数的注释代码

/* A background append only file rewriting (BGREWRITEAOF) terminated its work.
 * Handle this. */
void backgroundRewriteDoneHandler(int exitcode, int bysignal) {
    if (!bysignal && exitcode == 0) {//子进程退出状态正确
        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. */
        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;
        }
        //处理server.aof_rewrite_buf_blocks中DIFF数据
        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.
         *
         * 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) {
            /* 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". */
        //把临时文件改名为正常的AOF文件名。由于当前oldfd已经指向这个之前的正常文件名的文件，
        //所以当前不会造成unlink操作，得等那个oldfd被close的时候，内核判断该文件没有指向了，就删除之。
        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;
        }
        //如果AOF关闭了，那只要处理新文件，直接关闭这个新的文件即可
        //但是这里会不会导致服务器卡呢?这个newfd应该是临时文件的最后一个fd了，不会的，
        //因为这个文件在本函数不会写入数据，因为stopAppendOnly函数会清空aof_rewrite_buf_blocks列表。
        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;
            //指向新的fd，此时这个fd由于上面的rename语句存在，已经为正常aof文件名
            server.aof_fd = newfd;
            //fsync到硬盘
            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. */
            //rewrite得到的肯定是最新的数据，所以aof_buf中的数据没有意义，直接清空
            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 */
        //下面判断是否需要打开AOF，比如bgrewriteaofCommand就不需要打开AOF。
        if (server.aof_state == REDIS_AOF_WAIT_REWRITE)
            server.aof_state = REDIS_AOF_ON;

        /* Asynchronously close the overwritten AOF. */
        //让后台线程去关闭这个旧的AOF文件FD，只要CLOSE就行，会自动unlink的，因为上面已经有rename
        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);
    } 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);
    }

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;
}

至此，AOF数据持久化已经全部结束了，剩下的就是一些细节的处理，以及一些Linux库函数的理解，对于rename、unlink、wait3等库函数的深入认识就去问Google吧。

小结

Redis AOF数据持久化的实现机制通过三篇文章基本上比较详细的分析了，但这只是从代码层面去看AOF，对于AOF持久化的优缺点网上有很多分析，Redis的官方网站也有英文介绍，Redis的数据持久化还有一种方法叫RDB，更多RDB的内容等下次再分析。感谢此篇博客给我在理解Redis AOF数据持久化方面的巨大帮助，http://chenzhenianqing.cn/articles/786.html，此篇博客对AOF的分析十分的详细。