1. 什么是DistCp

　　DistCp(分布式拷贝)是用于大规模集群内部和集群之间拷贝的工具。它使用Map/Reduce实现文件分发，错误处理和恢复，以及报告生成。它把文件和目录的列表作为map任务的输入，每个任务会完成源列表中部分文件的拷贝。由于使用了Map/Reduce方法，这个工具在语义和执行上都会有特殊的地方。

1.1 DistCp使用的注意事项

　　1. DistCp会尝试着均分需要拷贝的内容，这样每个map拷贝差不多相等大小的内容。但因为文件是最小的拷贝粒度，所以配置增加同时拷贝（如map）的数目不一定会增加实际同时拷贝的数目以及总吞吐量。

　　2. 如果没使用-m选项，DistCp会尝试在调度工作时指定map的数据为 min (total_bytes / bytes.per.map, 20 * num_task_trackers)，其中bytes.per.map默认是256MB。

　　3. 建议对于长时间运行或定期运行的作业，根据源和目标集群大小、拷贝数量大小以及带宽调整map的数目。

　　4. 对于不同Hadoop版本间的拷贝，用户应该使用HftpFileSystem。这是一个只读文件系统，所以DistCp必须运行在目标端集群上（更确切的的说是能够写入目标集群的TaskTracker上）。源的格式是 hftp://<dfs.http.address>/<path> （默认情况dfs.http.address是 <namenode>:50070）。

2. Hadoop DistCp的api使用

[root@node105 ~]# hadoop distcpusage: distcp OPTIONS [source_path...] 
     
                    OPTIONS -append                       Reuse existing data in target files and                               append new data to them if possible -async                        Should distcp execution be blocking -atomic                       Commit all changes or none -bandwidth 
      
                     Specify bandwidth per map in MB -blocksperchunk 
       
                 If set to a positive value, fileswith more                               blocks than this value will be split into                               chunks of 
        
          blocks to be                               transferred in parallel, and reassembled on                               the destination. By default,                               
         
           is 0 and the files will be                               transmitted in their entirety without                               splitting. This switch is only applicable                               when the source file system implements                               getBlockLocations method and the target                               file system implements concat method -copybuffersize 
          
            Size of the copy buffer to use. By default 
           
             is 8192B. -delete Delete from target, files missing in source -diff 
            
              Use snapshot diff report to identify the difference between source and target -f 
             
               List of files that need to be copied -filelimit 
              
                (Deprecated!) Limit number of files copied to <= n -filters 
               
                 The path to a file containing a list of strings for paths to be excluded from the copy. -i Ignore failures during copy -log 
                
                  Folder on DFS where distcp execution logs are saved -m 
                 
                   Max number of concurrent maps to use for copy -mapredSslConf 
                  
                    Configuration for ssl config file, to use with hftps://. Must be in the classpath. -numListstatusThreads 
                   
                     Number of threads to use for building file listing (max 40). -overwrite Choose to overwrite target files unconditionally, even if they exist. -p 
                    
                      preserve status (rbugpcaxt)(replication, block-size, user, group, permission, checksum-type, ACL, XATTR, timestamps). If -p is specified with no 
                     
                      , then preserves replication, block size, user, group, permission, checksum type and timestamps. raw.* xattrs are preserved when both the source and destination paths are in the /.reserved/raw hierarchy (HDFS only). raw.* xattrpreservation is independent of the -p flag. Refer to the DistCp documentation for more details. -rdiff 
                      
                        Use target snapshot diff report to identify changes made on target -sizelimit 
                       
                         (Deprecated!) Limit number of files copied to <= n bytes -skipcrccheck Whether to skip CRC checks between source and target paths. -strategy 
                        
                          Copy strategy to use. Default is dividing work based on file sizes -tmp 
                         
                           Intermediate work path to be used for atomic commit -update Update target, copying only missingfiles or directories
                         
                        
                       
                      
                     
                    
                   
                  
                 
                
               
              
             
            
           
          
         
        
       
      
     

3. 测试用例

　　1. 查看将要迁移的目标文件

[root@calculation101 ~]# hdfs dfs -du -h /test/2018/10/

　　2. 创建新集群的测试目录：

[hdfs@node105 root]$ [hdfs@node105 root]$ hdfs dfs -mkdir -p /yangjianqiu/data/[hdfs@node105 root]$ [hdfs@node105 root]$ hdfs dfs -chown -R root:root  /yangjianqiu/data/  [hdfs@node105 root]$ [hdfs@node105 root]$ exit exit[root@node105 ~]# [root@node105 ~]# hdfs dfs -ls /yangjianqiuFound 1 itemsdrwxr-xr-x   - root root          0 2018-10-29 03:29 /yangjianqiu/data

　　2. 开始迁移数据I并记录日志以及迁移数据所用时间：

[root@node105 ~]# mkdir /yangjianqiu[root@node105 ~]# [root@node105 ~]# [root@node105 ~]# nohup time hadoop distcp hdfs://calculation101:8020/test/2018/10/23 hdfs://node105:8020/yangjianqiu/data >> /yangjianqiu/distcp.log 2>&1 & [1] 11125 [root@node105 ~]# [root@node105 ~]# jobs [1]+ Running nohup time hadoop distcp hdfs://calculation101:8020/test/2018/10/23 hdfs://node105:8020/yangjianqiu/data >> /yangjianqiu/distcp.log 2>&1 &

4. 应用程序调用distcp接口

总结

【参考资料】

 Hive 数据 bulkload 导入 HBase

  hadoop跨集群之间迁移hive数据

 hadoop 集群跨版本数据迁移

 DistCp between HA clusters

  Copying Cluster Data Using DistCp

 Java Code Examples for org.apache.hadoop.tools.DistCp

 HDFS集群PB级数据迁移方案-DistCp生产环境实操篇