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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