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https://issues.apache.org/jira/browse/HDFS-9426?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15004229#comment-15004229
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Kihwal Lee commented on HDFS-9426:
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bq. NN might have to check the DN version and return different
rollingUpgradeInfo.
In order to maintain the protocol semantics, it might be better to set the RU
state to null and return another one in a new optional field for newer datanode
to use.
> 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
>
> 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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