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https://issues.apache.org/jira/browse/HDFS-15?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12745313#action_12745313
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dhruba borthakur commented on HDFS-15:
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> A different queue (neededReplicationsForRacks) is maintained for blocks which
> do not have sufficient rac
There was a time when the Namenode was littered with plenty of adhoc data
structures, each for its own purpose. There was an effort to consolidate the
functionality of these data structures into a smaller set. I am not against
this patch, but is it really difficult to integrate this new data structure
into neededReplication as explained in your first proposal?
> 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
>
> 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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