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https://issues.apache.org/jira/browse/HDFS-1508?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12965599#action_12965599
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Konstantin Shvachko commented on HDFS-1508:
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> Ops wants to do a checkpoint without the BNN/SNN (say it is down) and is 
> willing to make the NN unresponsive for a short while.

So they set it into safe mode, saveNamespace, then leave safe mode. A few 
seconds wont make a difference.

> Ability to do savenamespace without being in safemode
> -----------------------------------------------------
>
>                 Key: HDFS-1508
>                 URL: https://issues.apache.org/jira/browse/HDFS-1508
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: name-node
>            Reporter: dhruba borthakur
>            Assignee: dhruba borthakur
>         Attachments: savenamespaceWithoutSafemode.txt
>
>
> In the current code, the administrator can run savenamespace only after 
> putting the namenode in safemode. This means that applications that are 
> writing to HDFS encounters errors because the NN is in safemode. We would 
> like to allow saveNamespace even when the namenode is not in safemode.
> The savenamespace command already acquires the FSNamesystem writelock. There 
> is no need to require that the namenode is in safemode too.

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