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From "Yongjun Zhang (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HDFS-7235) Can not decommission DN which has invalid block due to bad disk
Date Sat, 11 Oct 2014 04:20:33 GMT
Yongjun Zhang created HDFS-7235:

             Summary: Can not decommission DN which has invalid block due to bad disk
                 Key: HDFS-7235
                 URL: https://issues.apache.org/jira/browse/HDFS-7235
             Project: Hadoop HDFS
          Issue Type: Bug
          Components: datanode, namenode
    Affects Versions: 2.6.0
            Reporter: Yongjun Zhang
            Assignee: Yongjun Zhang

When to decommission a DN, the process hangs. 

What happens is, when NN chooses a replica as a source to replicate data on the to-be-decommissioned
DN to other DNs, it favors choosing this DN to-be-decommissioned as the source of transfer
(see BlockManager.java).  However, because of the bad disk, the DN would detect the source
block to be transfered as invalidBlock with the following logic in FsDatasetImpl.java:

/** Does the block exist and have the given state? */
  private boolean isValid(final ExtendedBlock b, final ReplicaState state) {
    final ReplicaInfo replicaInfo = volumeMap.get(b.getBlockPoolId(), 
    return replicaInfo != null
        && replicaInfo.getState() == state
        && replicaInfo.getBlockFile().exists();

The reason that this method returns false (detecting invalid block) is because the block file
doesn't exist in this case. 

The key issue we found here is, after DN detects an invalid block for the above reason, it
doesn't report the invalid block back to NN, thus NN doesn't know that the block is corrupted,
and keeps sending the data transfer request to the same DN to be decommissioned, again and
again. This caused an infinite loop, so the decommission process hangs.

Thanks [~qwertymaniac] for reporting the issue and initial analysis.

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