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https://issues.apache.org/jira/browse/HDFS-7230?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14170306#comment-14170306
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Tsz Wo Nicholas Sze commented on HDFS-7230:
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Same as downgrade, rolling downgrade requires the same NAMENODE_LAYOUT_VERSION
and the same DATANODE_LAYOUT_VERSION. Although there is no layout change, a
cluster may not be downgraded using the same rolling upgrade procedure since
protocols may change in a backward compatible manner but not forward
compatible, i.e. old DNs can talk to the new NNs but new DNs may not be able to
talk the old NNs.
> Support rolling downgrade
> -------------------------
>
> Key: HDFS-7230
> URL: https://issues.apache.org/jira/browse/HDFS-7230
> Project: Hadoop HDFS
> Issue Type: Improvement
> Reporter: Tsz Wo Nicholas Sze
> Assignee: Tsz Wo Nicholas Sze
>
> HDFS-5535 made a lot of improvement on rolling upgrade. It also added the
> cluster downgrade feature. However, the downgrade described in HDFS-5535
> requires cluster downtime. In this JIRA, we discuss how to do rolling
> downgrade, i.e. downgrade without downtime.
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