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https://issues.apache.org/jira/browse/HDFS-3091?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13245563#comment-13245563
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Tsz Wo (Nicholas), SZE commented on HDFS-3091:
----------------------------------------------
@Zhanwei, let me reply you on HDFS-3179.
> Update the usage limitations of ReplaceDatanodeOnFailure policy in the config
> description for the smaller clusters.
> -------------------------------------------------------------------------------------------------------------------
>
> Key: HDFS-3091
> URL: https://issues.apache.org/jira/browse/HDFS-3091
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: data-node, hdfs client, name-node
> Affects Versions: 0.23.0, 0.24.0
> Reporter: Uma Maheswara Rao G
> Assignee: Tsz Wo (Nicholas), SZE
> Fix For: 2.0.0
>
> Attachments: h3091_20120319.patch
>
>
> When verifying the HDFS-1606 feature, Observed couple of issues.
> Presently the ReplaceDatanodeOnFailure policy satisfies even though we dont
> have enough DN to replcae in cluster and will be resulted into write failure.
> {quote}
> 12/03/13 14:27:12 WARN hdfs.DFSClient: DataStreamer Exception
> java.io.IOException: Failed to add a datanode: nodes.length !=
> original.length + 1, nodes=[xx.xx.xx.xx:50010], original=[xx.xx.xx.xx1:50010]
> at
> org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer.findNewDatanode(DFSOutputStream.java:778)
> at
> org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer.addDatanode2ExistingPipeline(DFSOutputStream.java:834)
> at
> org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer.setupPipelineForAppendOrRecovery(DFSOutputStream.java:930)
> at
> org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer.processDatanodeError(DFSOutputStream.java:741)
> at
> org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer.run(DFSOutputStream.java:416)
> {quote}
> Lets take some cases:
> 1) Replication factor 3 and cluster size also 3 and unportunately pipeline
> drops to 1.
> ReplaceDatanodeOnFailure will be satisfied because *existings(1)<=
> replication/2 (3/2==1)*.
> But when it finding the new node to replace obiously it can not find the new
> node and the sanity check will fail.
> This will be resulted to Wite failure.
> 2) Replication factor 10 (accidentally user sets the replication factor to
> higher value than cluster size),
> Cluser has only 5 datanodes.
> Here even if one node fails also write will fail with same reason.
> Because pipeline max will be 5 and killed one datanode, then existings will
> be 4
> *existings(4)<= replication/2(10/2==5)* will be satisfied and obiously it
> can not replace with the new node as there is no extra nodes exist in the
> cluster. This will be resulted to write failure.
> 3) sync realted opreations also fails in this situations ( will post the
> clear scenarios)
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