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https://issues.apache.org/jira/browse/HDFS-5526?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13826825#comment-13826825
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Tsz Wo (Nicholas), SZE commented on HDFS-5526:
----------------------------------------------

Kihwal, you are right that there is no backup copy of ./current/VERSION and so 
it cannot be rolled back.

> Datanode cannot roll back to previous layout version
> ----------------------------------------------------
>
>                 Key: HDFS-5526
>                 URL: https://issues.apache.org/jira/browse/HDFS-5526
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: datanode
>            Reporter: Tsz Wo (Nicholas), SZE
>            Priority: Blocker
>
> Current trunk layout version is -48.
> Hadoop v2.2.0 layout version is -47.
> If a cluster is upgraded from v2.2.0 (-47) to trunk (-48), the datanodes 
> cannot start with -rollback.  It will fail with IncorrectVersionException.



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