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From "Tsz Wo (Nicholas), SZE (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-3091) Failed to add new DataNode in pipeline and will be resulted into write failure.
Date Thu, 15 Mar 2012 19:00:38 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3091?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13230426#comment-13230426

Tsz Wo (Nicholas), SZE commented on HDFS-3091:

Hi Uma,

First of all, thanks for testing it.

I would say the failures are expected.  The feature is to guarantee the number of replicas
that the user is asking.  However, the cluster is too small that the guarantee is impossible.
 It makes sense to fail the write requests.

Note that policy is a client side configuration.  The user could set the policy to NEVER.
 For the 3-node case, the admin should disable the feature (or set the policy to NEVER in
the default conf.)

Does it make sense to you?
> 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=[],
>         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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