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https://issues.apache.org/jira/browse/HDFS-9313?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14975592#comment-14975592
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Walter Su commented on HDFS-9313:
---------------------------------
I'm ok that adding a {{null}} check. However, I don't think it's enough to
address the scenario here, In the test case, you added 1 SSD + 3 DISKs. As you
said in the patch,
{code}
1040 // In this case,
1041 // no replica can't be chosen as the excessive replica as
1042 // chooseReplicasToDelete only considers storages[4] and
storages[5] that
1043 // are the same rack. But neither's storage type is SSD.
{code}
If we choose nothing, the replica on SSD won't be deleted. And I remember,
{{Mover}} won't do it neither, since the existings contains the expected.
Instead of choosing nothing, we should choose the SSD, since the remaining 3
DISKs are already on enough racks.
> Possible NullPointerException in BlockManager if no excess replica can be
> chosen
> --------------------------------------------------------------------------------
>
> Key: HDFS-9313
> URL: https://issues.apache.org/jira/browse/HDFS-9313
> Project: Hadoop HDFS
> Issue Type: Bug
> Reporter: Ming Ma
> Assignee: Ming Ma
> Attachments: HDFS-9313.patch
>
>
> HDFS-8647 makes it easier to reason about various block placement scenarios.
> Here is one possible case where BlockManager won't be able to find the excess
> replica to delete: when storage policy changes around the same time balancer
> moves the block. When this happens, it will cause NullPointerException.
> {noformat}
> java.lang.NullPointerException
> at
> org.apache.hadoop.hdfs.server.blockmanagement.BlockPlacementPolicy.adjustSetsWithChosenReplica(BlockPlacementPolicy.java:156)
> at
> org.apache.hadoop.hdfs.server.blockmanagement.BlockPlacementPolicyDefault.chooseReplicasToDelete(BlockPlacementPolicyDefault.java:978)
> {noformat}
> Note that it isn't found in any production clusters. Instead, it is found
> from new unit tests. In addition, the issue has been there before HDFS-8647.
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