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https://issues.apache.org/jira/browse/HDFS-15738?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17252536#comment-17252536
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Janus Chow commented on HDFS-15738:
-----------------------------------
Sure and thanks for the reply.
> Forbid the transition to Observer state when NameNode is in StartupSafeMode
> ---------------------------------------------------------------------------
>
> Key: HDFS-15738
> URL: https://issues.apache.org/jira/browse/HDFS-15738
> Project: Hadoop HDFS
> Issue Type: Improvement
> Reporter: Janus Chow
> Assignee: Janus Chow
> Priority: Major
> Attachments: HDFS-15738.001.patch
>
>
> Currently when a _getBlockLocation_ request comes to an Observer Namenode
> which is in safemode, NameNode will have a check that if the result is empty,
> it will reply to the client with a _RetriableException_, noting the client to
> retry the request later.
> And If the Observer Namenode is in startup safe mode, the client would have
> to wait for the Observer NameNode to leave the safe mode. For a big cluster,
> it may cause a long time of waiting for the client. In our cluster, we met
> this problem, and the client needs to wait for about 30 minutes before the
> service back to normal.
> The reason for this situation is that the NameNode becomes the state of
> Observer when it's still in safe mode getting Datanode's block reports. And
> here are two solutions for this issue:
> # Throw _ObserverRetryOnActiveException_ when the Observer NameNode is in
> startup safe mode, redirecting the user's requests to active NN.
> # Forbid the transition to Observer state when the cluster maintainer is
> trying to do the transition operation.
> We choose the second solution because the first one would abet the bad
> operation of transition NN to Observers while it's not ready for real service.
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