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https://issues.apache.org/jira/browse/HDFS-15?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12739821#action_12739821
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dhruba borthakur commented on HDFS-15:
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Does this problem exist (or have been observed) on 0.20?
> 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: Hairong Kuang
> Priority: Critical
>
> 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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