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                            ## 代碼已經上傳github
[hadoop-train](https://github.com/yijia1992/hadooptrainv2)

## hdfs寫數據流程
![](https://img.kancloud.cn/62/f6/62f638fd4df84f376815201c9d51432c_1124x510.png)

## hdfs讀數據流程
![](https://img.kancloud.cn/33/50/335044484e11ba879abb6d81afef5967_988x432.png)

## hdfs元數據管理
* 元數據：HDFS的目錄結構以及每個文件的BLOCK信息(id，副本系數、block存放在哪個DN上)
* 存在什么地方：對應配置 ${hadoop.tmp.dir}/name/......
![](https://img.kancloud.cn/b0/2c/b02c1f8952c47a5717b10a4c822042da_963x488.png)

## 安全模式
> 啟動時namenode先進入安全模式檢測datanode是否完好。檢測完畢后退出安全模式。在安全模式中是不可以讀寫的。
                    
                
        
                
            
        
                
    
                
    
                
        
                
    
                

                







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