[jira] [Created] (HDFS-7876) DataNodes start to scan blocks earlier

["Xinwei Qin (JIRA)" <ji...@apache.org>]

Xinwei Qin  created HDFS-7876:
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             Summary: DataNodes start to scan blocks earlier
                 Key: HDFS-7876
                 URL: https://issues.apache.org/jira/browse/HDFS-7876
             Project: Hadoop HDFS
          Issue Type: Improvement
          Components: datanode, namenode
    Affects Versions: 3.0.0
            Reporter: Xinwei Qin 
            Assignee: Xinwei Qin 


When Hadoop cluster restarts, DataNodes will scan local blocks, and report this infomation to NameNode. DataNodes start to scan local blocks after obtaining the NamespaceInfo from NameNode via RPC call versionRequest(), which needs the establishment of NameNode RPC server. 

Now, the RPC server will not be created and started until the completion of loading FsImage. So, DataNodes cannot start to scan blocks immediately, and must wait for NameNode to load FsImage. This will cause time wasting of DataNode when the FsImage is very large. 

Since the RPC server has very little dependence of FsImage, and the NamespaceInfo (namespaceID, clustered, blockpoolID, cTime, etc.) can be constructed from VERSION file, we can create and start RPC server before loading FsImage, so that DataNodes can get NamespaceInfo from NameNode via RPC call as soon as possible, and start to scan blocks earlier, which will shorten restart time.



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