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From "Todd Lipcon (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-2179) HA: namenode fencing mechanism
Date Thu, 04 Aug 2011 18:58:27 GMT

    [ https://issues.apache.org/jira/browse/HDFS-2179?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13079530#comment-13079530
] 

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

Yes, sorry for not running test-patch before. I just ran it and it pointed out that I was
missing the license on one of the new test files as well. Aside from that, it passes.

As for why it goes in the NN package instead of common, I think it's better to start with
building things specific to our current use case. Then, if we have need of this code from
another spot (eg MapReduce HA) we can consider moving it to common. But let's not overly generalize
until we have to -- eg from what I've heard about MR HA, it stores all of its critical state
in ZooKeeper so this sort of fencing is not necessary.

> 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
>             Fix For: HA branch (HDFS-1623)
>
>         Attachments: hdfs-2179.txt, hdfs-2179.txt, hdfs-2179.txt
>
>
> 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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