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Tsz Wo (Nicholas), SZE commented on HDFS-1480: ---------------------------------------------- If {{numOfResults==2}} and two existing replicas are in the same rack, we are going to choose a remote node as shown below. {code} //BlockPlacementPolicyDefault.chooseTarget(..) //line 194 if (numOfResults <= 2) { if (clusterMap.isOnSameRack(results.get(0), results.get(1))) { chooseRemoteRack(1, results.get(0), excludedNodes, blocksize, maxNodesPerRack, results); } else ... {code} > All replicas for a block with repl=2 end up in same rack > -------------------------------------------------------- > > Key: HDFS-1480 > URL: https://issues.apache.org/jira/browse/HDFS-1480 > Project: Hadoop HDFS > Issue Type: Bug > Components: name-node > Affects Versions: 0.20.2 > Reporter: T Meyarivan > > 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: > {code} > int maxNodesPerRack = > (totalNumOfReplicas-1)/clusterMap.getNumOfRacks()+2; > {code} > snippet-02: > {code} > case 2: > if (clusterMap.isOnSameRack(results.get(0), results.get(1))) { > chooseRemoteRack(1, results.get(0), excludedNodes, > blocksize, maxNodesPerRack, results); > } else if (newBlock){ > chooseLocalRack(results.get(1), excludedNodes, blocksize, > maxNodesPerRack, results); > } else { > chooseLocalRack(writer, excludedNodes, blocksize, > maxNodesPerRack, results); > } > if (--numOfReplicas == 0) { > break; > } > {code} > snippet-03: > {code} > do { > DatanodeDescriptor[] selectedNodes = > chooseRandom(1, nodes, excludedNodes); > if (selectedNodes.length == 0) { > throw new NotEnoughReplicasException( > "Not able to place enough > replicas"); > } > result = (DatanodeDescriptor)(selectedNodes[0]); > } while(!isGoodTarget(result, blocksize, maxNodesPerRack, results)); > {code} -- This message is automatically generated by JIRA. For more information on JIRA, see: http://www.atlassian.com/software/jira