[jira] [Created] (HDFS-15417) Lazy get the datanode report for federation WebHDFS operations

["Ye Ni (Jira)" <ji...@apache.org>]

Ye Ni created HDFS-15417:
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             Summary: Lazy get the datanode report for federation WebHDFS operations
                 Key: HDFS-15417
                 URL: https://issues.apache.org/jira/browse/HDFS-15417
             Project: Hadoop HDFS
          Issue Type: Improvement
          Components: federation, rbf, webhdfs
            Reporter: Ye Ni


*Why*
For WebHDFS CREATE, OPEN, APPEND and GETFILECHECKSUM operations, router or namenode needs to get the datanodes where the block is located, then redirect the request to one of the datanodes.

However, this chooseDatanode action in router is much slower than namenode, which directly affects the WebHDFS operations above.

For namenode WebHDFS, it normally takes tens of milliseconds, while router always takes more than 2 seconds.

*How*
Only get the datanode report when necessary in router. It is a very expense operation where all the time is spent on.

This is only need when we want to exclude some datanodes or find a random datanode for CREATE.



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