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Date: Tue, 26 Oct 2010 19:56:26 -0400 (EDT)
From: "T Meyarivan (JIRA)" <jira@apache.org>
To: hdfs-issues@hadoop.apache.org
Subject: [jira] Commented: (HDFS-1480) All replicas for a block end up in
 same rack
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    [ https://issues.apache.org/jira/browse/HDFS-1480?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12925206#action_12925206 ] 

T Meyarivan commented on HDFS-1480:
-----------------------------------

[1] verified via "dfsadmin -report" that rack id etc is being computed correctly for all datanodes
[2] number of such replicas doesn't seem to be reducing 
[3] balancer is/was running

-

[A] isGoodTarget() is broken for repl=2 ?
[B] 'hadoop fsck ...' uses slightly different logic to determine whether a block is correctly replicated ?

--


> All replicas for a block end up in same rack
> --------------------------------------------
>
>                 Key: HDFS-1480
>                 URL: https://issues.apache.org/jira/browse/HDFS-1480
>             Project: Hadoop HDFS
>          Issue Type: Bug
>            Reporter: T Meyarivan
>            Priority: Minor
>
> It appears that all replicas of a block can end up in the same rack. The likelihood of such replicas seems to be directly related to decommissioning of nodes. 
> Post rolling OS upgrade (decommission 3-10% of nodes, re-install etc, add them back) of a running cluster, all replicas of about 0.16% of blocks ended up in the same rack.
> Hadoop Namenode UI etc doesn't seem to know about such incorrectly replicated blocks. "hadoop fsck .." does report that the blocks must be replicated on additional racks.
> Looking at ReplicationTargetChooser.java, following seem suspect:
> snippet-01:
> """
>     int maxNodesPerRack =
>       (totalNumOfReplicas-1)/clusterMap.getNumOfRacks()+2;
> """
> snippet-02:
> """
>     if (counter>maxTargetPerLoc) {
>       logr.debug("Node "+NodeBase.getPath(node)+
>                 " is not chosen because the rack has too many chosen nodes");
>       return false;
>     }
> """
> snippet-03:
> """
>       default:
>         chooseRandom(numOfReplicas, NodeBase.ROOT, excludedNodes,
>                      blocksize, maxNodesPerRack, results);
>       }
> """
> --

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