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https://issues.apache.org/jira/browse/HDFS-6800?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14113081#comment-14113081
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James Thomas commented on HDFS-6800:
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Right, the scenario I'm talking about is the following:
1) We start rolling upgrade and restart some DNs with a new DN layout version,
so previous directories are created
2) Some of the restarted DNs send block reports to the NN, and, since
{{isUpgradeFinalized}} is true, it responds with a FinalizeStatus that
indicates that the upgrade has been finalized (see the code in
{{NameNodeRpcServer#blockReport}}). The DNs delete their previous directories.
3) We now attempt to roll back the rolling upgrade, but the aforementioned DNs
cannot be rolled back because their previous directories have been deleted.
> Determine how Datanode layout changes should interact with rolling upgrade
> --------------------------------------------------------------------------
>
> Key: HDFS-6800
> URL: https://issues.apache.org/jira/browse/HDFS-6800
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: datanode
> Affects Versions: 2.6.0
> Reporter: Colin Patrick McCabe
> Assignee: James Thomas
> Attachments: HDFS-6800.2.patch, HDFS-6800.3.patch, HDFS-6800.4.patch,
> HDFS-6800.5.patch, HDFS-6800.patch
>
>
> We need to handle attempts to rolling-upgrade the DataNode to a new storage
> directory layout.
> One approach is to disallow such upgrades. If we choose this approach, we
> should make sure that the system administrator gets a helpful error message
> and a clean failure when trying to use rolling upgrade to a version that
> doesn't support it. Based on the compatibility guarantees described in
> HDFS-5535, this would mean that *any* future DataNode layout changes would
> require a major version upgrade.
> Another approach would be to support rolling upgrade from an old DN storage
> layout to a new layout. This approach requires us to change our
> documentation to explain to users that they should supply the {{\-rollback}}
> command on the command-line when re-starting the DataNodes during rolling
> rollback. Currently the documentation just says to restart the DataNode
> normally.
> Another issue here is that the DataNode's usage message describes rollback
> options that no longer exist. The help text says that the DN supports
> {{\-rollingupgrade rollback}}, but this option was removed by HDFS-6005.
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