if (!strcasecmp(argv[0],"appendonly") && argc == 2) {
	int yes;
	if ((yes = yesnotoi(argv[1])) == -1) {
		err = "argument must be 'yes' or 'no'"; goto loaderr;
	}
	server.aof_state = yes ? REDIS_AOF_ON : REDIS_AOF_OFF;
}

if (!strcasecmp(argv[0],"appendfsync") && argc == 2) {//AOF fsync策略
	if (!strcasecmp(argv[1],"no")) {
		server.aof_fsync = AOF_FSYNC_NO;
	} else if (!strcasecmp(argv[1],"always")) {
		server.aof_fsync = AOF_FSYNC_ALWAYS;
	} else if (!strcasecmp(argv[1],"everysec")) {
		server.aof_fsync = AOF_FSYNC_EVERYSEC;
	} else {
		err = "argument must be 'no', 'always' or 'everysec'";
		goto loaderr;
	}
}

if (!strcasecmp(argv[0],"no-appendfsync-on-rewrite") && argc == 2) {
	if ((server.aof_no_fsync_on_rewrite= yesnotoi(argv[1])) == -1) {
		err = "argument must be 'yes' or 'no'"; goto loaderr;
	}
}

//如果不支持fsync，或者aof rdb子进程正在运行，那么直接返回，
//但是数据已经写到aof文件中，只是没有刷新到硬盘
if (server.aof_no_fsync_on_rewrite &&
	(server.aof_child_pid != -1 || server.rdb_child_pid != -1))
		return;

struct redisServer {
	…………
	
    /* AOF persistence */
    int aof_state;                  /* REDIS_AOF_(ON|OFF|WAIT_REWRITE) */
    int aof_fsync;                  /* Kind of fsync() policy */
    char *aof_filename;             /* Name of the AOF file */
    int aof_no_fsync_on_rewrite;    /* Don't fsync if a rewrite is in prog. */
    int aof_rewrite_perc;           /* Rewrite AOF if % growth is > M and... */
    off_t aof_rewrite_min_size;     /* the AOF file is at least N bytes. */ aof文件最小字节数
    off_t aof_rewrite_base_size;    /* AOF size on latest startup or rewrite. */ 上一次aof文件的大小
    off_t aof_current_size;         /* AOF current size. */ 当前aof文件的大小
    int aof_rewrite_scheduled;      /* Rewrite once BGSAVE terminates. */
    pid_t aof_child_pid;            /* PID if rewriting process */ 子进程的pid
    list *aof_rewrite_buf_blocks;   /* Hold changes during an AOF rewrite. */
    sds aof_buf;      /* AOF buffer, written before entering the event loop */
    int aof_fd;       /* File descriptor of currently selected AOF file */ aof文件描述符
    int aof_selected_db; /* Currently selected DB in AOF */
    time_t aof_flush_postponed_start; /* UNIX time of postponed AOF flush */
    time_t aof_last_fsync;            /* UNIX time of last fsync() */
    time_t aof_rewrite_time_last;   /* Time used by last AOF rewrite run. */
    time_t aof_rewrite_time_start;  /* Current AOF rewrite start time. */
    int aof_lastbgrewrite_status;   /* REDIS_OK or REDIS_ERR */
    unsigned long aof_delayed_fsync;  /* delayed AOF fsync() counter */
    int aof_rewrite_incremental_fsync;/* fsync incrementally while rewriting? */
	
	…………
}

server.aof_state = REDIS_AOF_OFF;            //AOF文件是否开启
server.aof_fsync = REDIS_DEFAULT_AOF_FSYNC;  //fsync策略，默认为每秒fsync
server.aof_no_fsync_on_rewrite = REDIS_DEFAULT_AOF_NO_FSYNC_ON_REWRITE;//aof rdb子进程运行中是否支持fsync，即写入AOF文件后，将数据刷新到硬盘
server.aof_rewrite_perc = REDIS_AOF_REWRITE_PERC; //自动rewrite增量值
server.aof_rewrite_min_size = REDIS_AOF_REWRITE_MIN_SIZE; //AOF文件最小字节数
server.aof_rewrite_base_size = 0;   //自动rewrite计算aof文件增量的基数，等于上一次aof文件的字节数
server.aof_rewrite_scheduled = 0;   //rewrite任务计划，当客户端发送bgrewriteaof指令，如果当前rewrite子进程正在执行，那么将客户端请求的bgrewriteaof变为计划任务，待AOF子进程结束后执行rewrite
server.aof_last_fsync = time(NULL); //最近fsync数据到硬盘时间
server.aof_rewrite_time_last = -1;  //rewrite持续的时间
server.aof_rewrite_time_start = -1; //rewrite开始的时间
server.aof_lastbgrewrite_status = REDIS_OK; //rewrite后的状态
server.aof_delayed_fsync = 0; //延迟fsync到硬盘的次数
server.aof_fd = -1;   //AOF文件描述符
server.aof_selected_db = -1; /* Make sure the first time will not match */
server.aof_flush_postponed_start = 0; //上次推迟fsync到硬盘的时间
server.aof_rewrite_incremental_fsync = REDIS_DEFAULT_AOF_REWRITE_INCREMENTAL_FSYNC;//rewrite AOF文件时是否采用增量式fsync