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https://issues.apache.org/jira/browse/HDFS-12638?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16202092#comment-16202092
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Daryn Sharp commented on HDFS-12638:
------------------------------------

It does differ from our case but likely has the same root cause.  The block was 
in your blocks map, but not in ours.  The block existed on your DN, but not 
ours.  The commonality is the block referenced a non-existent inode/collection.

> 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 
>
> 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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