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https://issues.apache.org/jira/browse/HDFS-15?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12745640#action_12745640
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Raghu Angadi commented on HDFS-15:
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I am in favor of  not having another list. I looked at the patch briefly, not 
sure if having extra set reduces new code.. Are you implying newer proposal has 
less changes? 

It is better to have 'neededReplicationList' to have all the block that need 
replication. May be 'UnderReplicatedBlocks' could be renamed. 

> All replicas of a block end up on only 1 rack
> ---------------------------------------------
>
>                 Key: HDFS-15
>                 URL: https://issues.apache.org/jira/browse/HDFS-15
>             Project: Hadoop HDFS
>          Issue Type: Bug
>            Reporter: Hairong Kuang
>            Assignee: Jitendra Nath Pandey
>            Priority: Critical
>         Attachments: HDFS-15.patch
>
>
> HDFS replicas placement strategy guarantees that the replicas of a block 
> exist on at least two racks when its replication factor is greater than one. 
> But fsck still reports that the replicas of some blocks  end up on one rack.
> The cause of the problem is that decommission and corruption handling only 
> check the block's replication factor but not the rack requirement. When an 
> over-replicated block loses a replica due to decomission, corruption, or 
> heartbeat lost, namenode does not take any action to guarantee that remaining 
> replicas are on different racks.
>  

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