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https://issues.apache.org/jira/browse/HDFS-1480?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13054217#comment-13054217
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Todd Lipcon commented on HDFS-1480:
-----------------------------------
There's definitely a bug still in trunk, here. Here's my analysis (in this case
with repl=3):
- Cluster has 2 racks, A and B, each with three nodes.
- Block initially written on A1, B1, B2
- Admin decommissions two of these nodes (let's say A1 and B1 but it doesn't
matter)
- in {{computeReplicationWorkForBlock}}, it calls {{chooseSourceDatanode}}
which populates {{containingNodes}} with all three nodes regardless of decom
status
- this is then passed through as {{chosenNodes}} in the call to {{chooseTarget}}
- maxNodesPerRack is then assigned (5-1)/2 + 2 = 4
- {{chooseTarget}} initializes {{result}} with these same three nodes
- {numOfResults}} is thus 3, and it calls {{chooseRandom}} rather than basing
its decision on local/remote racks
- {{chooseRandom}} is free to pick any two nodes since maxNodesPerRack is 4
Will attach a failing unit test momentarily
> 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}
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