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https://issues.apache.org/jira/browse/HDFS-3958?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13458528#comment-13458528
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Ivan Kelly commented on HDFS-3958:
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I guess an "upgrade" operation itself isn't really needed, as when you upgrade
the NN, it merges the editlog with the image. Then the upgraded NN can use what
looks like a newly formatted editlog. Doing this on a live system with multiple
possible writers to the editlog could be problematic though.
Snapshotting itself shouldn't be too difficult for us, as we just need to take
a snapshot of the zk tree at a certain znode version.
> Integrate upgrade/finalize/rollback with external journals
> ----------------------------------------------------------
>
> Key: HDFS-3958
> URL: https://issues.apache.org/jira/browse/HDFS-3958
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: name-node
> Affects Versions: 3.0.0
> Reporter: Todd Lipcon
>
> Currently the NameNode upgrade/rollback/finalize framework only supports
> local storage. With edits being stored in pluggable Journals, this could
> create certain difficulties - in particular, rollback wouldn't actually
> rollback the external storage to the old state.
> We should look at how to expose the right hooks to the external journal
> storage to snapshot/rollback/finalize.
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