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From "Uma Maheswara Rao G (Created) (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HDFS-3091) Failed to add new DataNode in pipeline and will be resulted into write failure.
Date Wed, 14 Mar 2012 05:37:43 GMT
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.

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)

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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