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https://issues.apache.org/jira/browse/HDFS-779?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12780228#action_12780228
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Owen O'Malley commented on HDFS-779:
------------------------------------

To address Allen's concern about small clusters, I'd suggest a minimum 
threshold of 25 nodes being down from the peak.

> Automatic move to safe-mode when cluster size drops
> ---------------------------------------------------
>
>                 Key: HDFS-779
>                 URL: https://issues.apache.org/jira/browse/HDFS-779
>             Project: Hadoop HDFS
>          Issue Type: New Feature
>          Components: name-node
>            Reporter: Owen O'Malley
>
> As part of looking at using Kerberos, we want to avoid the case where both 
> the primary (and optional secondary) KDC go offline causing a replication 
> storm as the DataNodes' service tickets time out and they lose the ability to 
> connect to the NameNode. However, this is a specific case of a more general 
> problem of loosing too many nodes too quickly. I think we should have an 
> option to go into safe mode if the cluster size goes down more than N% in 
> terms of DataNodes.

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