artAppendOnly(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 compare

Redis数据持久化机制AOF原理分析二(二)

Redis AOF rewrite机制的实现