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https://issues.apache.org/jira/browse/HDFS-6800?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14113276#comment-14113276
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James Thomas commented on HDFS-6800:
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A couple of notes:
1) We do not need to modify {{isUpgradeFinalized}} when rollback is initiated
on the NameNode because we assume that all of the DataNodes have been shut down
at this point, and they will be started up again with the {{-rollback}} flag,
so their previous directories will be immediately moved to current and we no
longer need to worry about them.
2) This patch doesn't address a possible NameNode crash between the point when
the rolling upgrade is started and when it is finalized. If this happens, when
the NameNode restarts {{isUpgradeFinalized}} will be set to true once again and
the DNs' previous directories will be deleted before upgrade finalization, as I
described above. I will try to post an updated patch addressing this case.
> 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-6800.6.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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