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 st

Redis数据持久化机制AOF原理分析二(一)

AOF rewrite的触发机制

接收到BGREWRITEAOF指令

Server自动对AOF进行rewrite

config set appendonly yes