Redis数据持久化机制AOF原理分析二 - 数据库编程

rogress 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.
         *

Redis数据持久化机制AOF原理分析二(五)