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https://issues.apache.org/jira/browse/HDFS-9314?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15024385#comment-15024385
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Junping Du commented on HDFS-9314:
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Let's keep BlockPlacementPolicyWithNodeGroup#pickupReplicaSet there, otherwise 
it will mess up block placement after deleting extra replicas. Actually, I 
think it is better to make BlockPlacementPolicyWithNodeGroup override 
verifyBlockPlacement() to check status according to its own policy. I will file 
a separated JIRA to track this.

> Improve BlockPlacementPolicyDefault's picking of excess replicas
> ----------------------------------------------------------------
>
>                 Key: HDFS-9314
>                 URL: https://issues.apache.org/jira/browse/HDFS-9314
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>            Reporter: Ming Ma
>            Assignee: Xiao Chen
>         Attachments: HDFS-9314.001.patch, HDFS-9314.002.patch, 
> HDFS-9314.003.patch, HDFS-9314.004.patch, HDFS-9314.005.patch, 
> HDFS-9314.006.patch
>
>
> The test case used in HDFS-9313 identified NullPointerException as well as 
> the limitation of excess replica picking. If the current replicas are on 
> {SSD(rack r1), DISK(rack 2), DISK(rack 3), DISK(rack 3)} and the storage 
> policy changes to HOT_STORAGE_POLICY_ID, BlockPlacementPolicyDefault's won't 
> be able to delete SSD replica.



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