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https://issues.apache.org/jira/browse/HDFS-3091?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13231824#comment-13231824
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Uma Maheswara Rao G commented on HDFS-3091:
-------------------------------------------
{quote}
Does it make sense to you?
{quote}
Yes, it make sense to me. There is no point of enabling this feature for
smaller clusters. This feature will give real advantages only when we go into
big clusters.
What if we log about the cluster size and can recommend to disable this feature
with smaller clusters? because by default this feature is enabled, when user
testing with 2-3 nodes clusters, this may fail due to pipeline failure and will
be resulted to write failure.
{quote}
Failed to add a datanode: nodes.length != original.length + 1,
nodes=[10.18.52.55:50010], original=[10.18.52.55:50010]
{quote}
for example:
Cluster size and replication factor nearly equal, there is no extra node
available in the cluster to add the new node into this pipeline. Hadoop
recommends to set dfs.client.block.write.replace-datanode-on-failure.policy to
NEVER for smaller clusters.
@liaowenrui, do you agree with Nicholas?
> Failed to add new DataNode in pipeline and will be resulted into write
> failure.
> -------------------------------------------------------------------------------
>
> Key: HDFS-3091
> URL: https://issues.apache.org/jira/browse/HDFS-3091
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: data-node, hdfs client, name-node
> Affects Versions: 0.23.0, 0.24.0
> Reporter: Uma Maheswara Rao G
>
> 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=[10.18.52.55:50010], original=[10.18.52.55:50010]
> 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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