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Date: Wed, 20 Jul 2011 23:11:57 +0000 (UTC)
From: "Todd Lipcon (JIRA)" <jira@apache.org>
To: hdfs-issues@hadoop.apache.org
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Subject: [jira] [Commented] (HDFS-2179) HA: namenode fencing mechanism
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    [ https://issues.apache.org/jira/browse/HDFS-2179?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13068702#comment-13068702 ] 

Todd Lipcon commented on HDFS-2179:
-----------------------------------

and one more open question I forgot: do we care about _read_ fencing? ie, is it OK if the old NN can for some number of seconds service reads which are no longer up-to-date? If so, storage fencing is necessary but not sufficient.

> HA: namenode fencing mechanism
> ------------------------------
>
>                 Key: HDFS-2179
>                 URL: https://issues.apache.org/jira/browse/HDFS-2179
>             Project: Hadoop HDFS
>          Issue Type: Sub-task
>          Components: name-node
>            Reporter: Todd Lipcon
>            Assignee: Todd Lipcon
>
> In an HA cluster, when there are two NNs, the invariant that only one NN is active at a time has to be preserved in order to prevent "split brain syndrome." Thus, when a standby NN is transition to "active" state during a failover, it needs to somehow _fence_ the formerly active NN to ensure that it can no longer perform edits. This JIRA is to discuss and implement NN fencing.

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