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Date: Mon, 13 Sep 2010 01:59:35 -0400 (EDT)
From: "dhruba borthakur (JIRA)" <jira@apache.org>
To: hdfs-issues@hadoop.apache.org
Subject: [jira] Commented: (HDFS-779) Automatic move to safe-mode when
 cluster size drops
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    [ https://issues.apache.org/jira/browse/HDFS-779?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12908630#action_12908630 ] 

dhruba borthakur commented on HDFS-779:
---------------------------------------

I forgot to mention the basic premise for *delaying* the replication event. When a catastrophic event happens, the replicas (and datanodes) are actually alive and n good health, it is just that they are unable to communicate with the namenode. So, immediate replication does not make sense. It is better to wait for some time to see if the existing replicas come back to life.

> 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
>            Assignee: dhruba borthakur
>
> 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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