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https://issues.apache.org/jira/browse/HDFS-4898?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13729489#comment-13729489
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Eric Sirianni commented on HDFS-4898:
-------------------------------------
Thanks Nicholas - yes, I will update the patch based on your suggestion.
> BlockPlacementPolicyWithNodeGroup.chooseRemoteRack() fails to properly
> fallback to local rack
> ---------------------------------------------------------------------------------------------
>
> Key: HDFS-4898
> URL: https://issues.apache.org/jira/browse/HDFS-4898
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: namenode
> Affects Versions: 1.2.0, 2.0.4-alpha
> Reporter: Eric Sirianni
> Assignee: Eric Sirianni
> Priority: Minor
>
> As currently implemented, {{BlockPlacementPolicyWithNodeGroup}} does not
> properly fallback to local rack when no nodes are available in remote racks,
> resulting in an improper {{NotEnoughReplicasException}}.
> {code:title=BlockPlacementPolicyWithNodeGroup.java}
> @Override
> protected void chooseRemoteRack(int numOfReplicas,
> DatanodeDescriptor localMachine, HashMap<Node, Node> excludedNodes,
> long blocksize, int maxReplicasPerRack, List<DatanodeDescriptor>
> results,
> boolean avoidStaleNodes) throws NotEnoughReplicasException {
> int oldNumOfReplicas = results.size();
> // randomly choose one node from remote racks
> try {
> chooseRandom(
> numOfReplicas,
> "~" +
> NetworkTopology.getFirstHalf(localMachine.getNetworkLocation()),
> excludedNodes, blocksize, maxReplicasPerRack, results,
> avoidStaleNodes);
> } catch (NotEnoughReplicasException e) {
> chooseRandom(numOfReplicas - (results.size() - oldNumOfReplicas),
> localMachine.getNetworkLocation(), excludedNodes, blocksize,
> maxReplicasPerRack, results, avoidStaleNodes);
> }
> }
> {code}
> As currently coded the {{chooseRandom()}} call in the {{catch}} block will
> never succeed as the set of nodes within the passed in node path (e.g.
> {{/rack1/nodegroup1}}) is entirely contained within the set of excluded nodes
> (both are the set of nodes within the same nodegroup as the node chosen first
> replica).
> The bug is that the fallback {{chooseRandom()}} call in the catch block
> should be passing in the _complement_ of the node path used in the initial
> {{chooseRandom()}} call in the try block (e.g. {{/rack1}}) - namely:
> {code}
> NetworkTopology.getFirstHalf(localMachine.getNetworkLocation())
> {code}
> This will yield the proper fallback behavior of choosing a random node from
> _within the same rack_, but still excluding those nodes _in the same
> nodegroup_
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