ZanderXu opened a new pull request, #4497:
URL: https://github.com/apache/hadoop/pull/4497

   ### Description of PR
   In our prod environment, 8020 Handler of Observer NameNode also blocked by 
EditlogTailer. 
   
   And EditlogTailer cost a long time for selecting InputStreams from 
Journalnode with holding the FSNLock. 
   
   During this period, 8020 handlers of Observer NameNode will be blocked by 
the FSN Lock.
   
   In theory, selecting inputstreams from JournalNode does not involve changing 
memory information in NameNode, so we can move the selecting out of the FSN 
Lock, and it can improve the throughput of Observer NameNode.
   
   After reading the relevant code in depth, I found that it is feasible to 
move the selecting inputStream out of the FSNLock.


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