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https://issues.apache.org/jira/browse/HDFS-12638?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16256293#comment-16256293
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Konstantin Shvachko commented on HDFS-12638:
--------------------------------------------

I think it's a blocker for all branches 2.8 and up. Even just removing that 
line {{toDelete.delete();}} would prevent crashing NameNode.

> NameNode exits due to ReplicationMonitor thread received Runtime exception in 
> ReplicationWork#chooseTargets
> -----------------------------------------------------------------------------------------------------------
>
>                 Key: HDFS-12638
>                 URL: https://issues.apache.org/jira/browse/HDFS-12638
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: hdfs
>    Affects Versions: 2.8.2
>            Reporter: Jiandan Yang 
>            Priority: Critical
>         Attachments: HDFS-12638-branch-2.8.2.001.patch, HDFS-12638.002.patch, 
> OphanBlocksAfterTruncateDelete.jpg
>
>
> Active NamNode exit due to NPE, I can confirm that the BlockCollection passed 
> in when creating ReplicationWork is null, but I do not know why 
> BlockCollection is null, By view history I found 
> [HDFS-9754|https://issues.apache.org/jira/browse/HDFS-9754] remove judging  
> whether  BlockCollection is null.
> NN logs are as following:
> {code:java}
> 2017-10-11 16:29:06,161 ERROR [ReplicationMonitor] 
> org.apache.hadoop.hdfs.server.blockmanagement.BlockManager: 
> ReplicationMonitor thread received Runtime exception.
> java.lang.NullPointerException
>         at 
> org.apache.hadoop.hdfs.server.blockmanagement.ReplicationWork.chooseTargets(ReplicationWork.java:55)
>         at 
> org.apache.hadoop.hdfs.server.blockmanagement.BlockManager.computeReplicationWorkForBlocks(BlockManager.java:1532)
>         at 
> org.apache.hadoop.hdfs.server.blockmanagement.BlockManager.computeReplicationWork(BlockManager.java:1491)
>         at 
> org.apache.hadoop.hdfs.server.blockmanagement.BlockManager.computeDatanodeWork(BlockManager.java:3792)
>         at 
> org.apache.hadoop.hdfs.server.blockmanagement.BlockManager$ReplicationMonitor.run(BlockManager.java:3744)
>         at java.lang.Thread.run(Thread.java:834)
> {code}



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