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https://issues.apache.org/jira/browse/HDFS-9426?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15004366#comment-15004366
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Kihwal Lee commented on HDFS-9426:
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I've also noticed that when 2.7.2rc0 namenode does RU finalization first and a
pre 2.7.2 datanode later upgrades, it deletes the ru marker file but does not
clear the trash. As far as I can see it stops populating the trash. I haven't
root-caused it yet.
> Rollingupgrade finalization is not backward compatible
> ------------------------------------------------------
>
> Key: HDFS-9426
> URL: https://issues.apache.org/jira/browse/HDFS-9426
> Project: Hadoop HDFS
> Issue Type: Bug
> Reporter: Kihwal Lee
> Priority: Blocker
> Attachments: HDFS-9426.branch-2.7.poc.patch, HDFS-9426.trunk.poc.patch
>
>
> After HDFS-7645, the namenode can return non-null {{rollingUpgradeInfo}} in
> heatbeat reponses. 2.7.1 or 2.6.x datanodes won't finalize the upgrade
> because it's not null.
> NN might have to check the DN version and return different
> {{rollingUpgradeInfo}}.
> HDFS-8656 recognized the compatibility issue of the changed semantics, but
> unfortunately did not address the semantics of the heartbeat response.
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