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https://issues.apache.org/jira/browse/HDFS-14074?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16926343#comment-16926343
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Zhankun Tang commented on HDFS-14074:
-------------------------------------

[~jojochuang], [~luguangyi], [~arp], Could you please update the release note? 
this is a blocker for the 3.1.3 release too. Thanks a lot.

> DataNode runs async disk checks  maybe  throws NullPointerException, and 
> DataNode failed to register to NameSpace.
> ------------------------------------------------------------------------------------------------------------------
>
>                 Key: HDFS-14074
>                 URL: https://issues.apache.org/jira/browse/HDFS-14074
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: hdfs
>    Affects Versions: 2.8.0, 3.0.0
>         Environment: hadoop-2.7.3, hadoop-2.8.0
>            Reporter: guangyi lu
>            Assignee: guangyi lu
>            Priority: Major
>              Labels: HDFS, HDFS-11114
>             Fix For: 3.3.0, 3.2.1, 3.1.3
>
>         Attachments: HDFS-14074-latest.patch, HDFS-14074.patch, 
> WechatIMG83.jpeg
>
>   Original Estimate: 48h
>  Remaining Estimate: 48h
>
> In ThrottledAsyncChecker class,it members of the completedChecks is 
> WeakHashMap, its definition is as follows:
>       this.completedChecks = new WeakHashMap<>();
> and one of its uses is as follows in schedule method:
>      if (completedChecks.containsKey(target)) {  
>       // here may be happen garbage collection,and result may be null.
>        final LastCheckResult<V> result = completedChecks.get(target);         
>  
>        final long msSinceLastCheck = timer.monotonicNow() - 
> result.completedAt;    
>        。。。。
> }
> after  "completedChecks.containsKey(target)",  may be happen garbage 
> collection,  and result may be null.
> the solution is:
> this.completedChecks = new ReferenceMap(1, 1);
> or
>  this.completedChecks = new HashMap<>();
>  



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