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https://issues.apache.org/jira/browse/HDFS-7230?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14170329#comment-14170329
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Tsz Wo Nicholas Sze commented on HDFS-7230:
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Here is the Rolling Downgrade procedure. Suppose a rolling upgrade is in
progress in a a HA cluster.
# Downgrade DNs
## Choose a small subset of datanodes (e.g. all datanodes under a particular
rack).
### Run "hdfs dfsadmin -shutdownDatanode <DATANODE_HOST:IPC_PORT> upgrade" to
shutdown one of the chosen datanodes.
### Run "hdfs dfsadmin -getDatanodeInfo <DATANODE_HOST:IPC_PORT>" to check and
wait for the datanode to shutdown.
### Downgrade and restart the datanode.
### Perform the above steps for all the chosen datanodes in the subset in
parallel.
## Repeat the above steps until all datanodes in the cluster are downgraded.
# Downgrade Active and Standby NNs: NN1 is active and NN2 is standby.
## Shutdown and downgrade NN2.
## Start NN2 as standby (the “-rollingUpgrade downgrade” option is not needed)
## Failover from NN1 to NN2 so that NN2 becomes active and NN1 becomes standby.
## Shutdown and downgrade NN1.
## Start NN1 as standby (the “-rollingUpgrade downgrade” option is not needed).
# Finalize
## Run "hdfs dfsadmin -rollingUpgrade finalize" to finalize the procedure.
> 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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