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https://issues.apache.org/jira/browse/HDFS-1271?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12884171#action_12884171
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Andrew Ryan commented on HDFS-1271:
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Operational note: We decommission nodes a lot (like, one or more per day).
Whenever we detect a failed component on a node that is going to necessitate
someone taking it down for repairs, we decommission as a preventative measure,
because our SiteOps team could do the repair at any time after that.
Occasionally I have seen nodes take hours or days or more to decommission, if
they are unlucky recipients of a block from a file from some very long-running
job.
IIRC it's not just NN restarts that are a problem, DN restarts also have the
same problem because when the DN comes up it will be denied communication by NN
and immediately shut down, even if it hasn't finished decom'ing. Then you have
to un-exclude and re-exclude to get the blocks flowing again. But at least
there with manual intervention you can get your blocks back.
> Decommissioning nodes not persisted between NameNode restarts
> -------------------------------------------------------------
>
> Key: HDFS-1271
> URL: https://issues.apache.org/jira/browse/HDFS-1271
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: name-node
> Reporter: Travis Crawford
>
> Datanodes in the process of being decomissioned should still be
> decomissioning after namenode restarts. Currently they are marked as dead
> after a restart.
> Details:
> Nodes can be safely removed from a cluster by marking them as decomissioned
> and waiting for their data to be replicated elsewhere. This is accomplished
> by adding a node to the filed referenced by dfs.hosts.excluded, then
> refreshing nodes.
> Decomissioning means block reports from the decomissioned datanode are no
> longer accepted by the namenode, meaning for decomissioning to occur the NN
> must have an existing block report. That is, a datanode can transition from:
> live --> decomissioning --> dead. Nodes can NOT transition from: dead -->
> decomissioning --> dead.
> Operationally this is problematic because intervention is required should the
> NN restart while nodes are decomissioning, meaning in-house administration
> tools must be more complex, or more likely admins have to babysit the
> decomissioning process.
> Someone more familiar with the code might have a better idea, but perhaps the
> first block report for dfs.hosts.excluded hosts should be accepted?
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