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From "Tsz Wo (Nicholas), SZE (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-4898) BlockPlacementPolicyWithNodeGroup.chooseRemoteRack() fails to properly fallback to local rack
Date Thu, 15 Aug 2013 05:09:49 GMT

    [ https://issues.apache.org/jira/browse/HDFS-4898?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13740680#comment-13740680
] 

Tsz Wo (Nicholas), SZE commented on HDFS-4898:
----------------------------------------------

Also committed the branch-1 patch.
                
> 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: Tsz Wo (Nicholas), SZE
>            Priority: Minor
>             Fix For: 1.3.0, 2.1.1-beta
>
>         Attachments: h4898_20130809_b-1.patch, h4898_20130809.patch
>
>
> 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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