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https://issues.apache.org/jira/browse/HDFS-5526?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13826661#comment-13826661
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Kihwal Lee commented on HDFS-5526:
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Well, it is being called for {{DataStorage}} when {{recoverTransitionRead()}}
is called for the first block pool that is being initialized.
{{DataStorage.doUpgrade()}} will simply write the new version and return. So
"previous" won't be created. In {{DataStorage.doRollback()}}, the comment says,
"Do nothing, if previous directory does not exist" and that's what it does.
> 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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