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Date: Wed, 25 Jul 2012 17:28:37 +0000 (UTC)
From: "Chao Shi (JIRA)" <jira@apache.org>
To: hdfs-issues@hadoop.apache.org
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Subject: [jira] [Commented] (HDFS-3077) Quorum-based protocol for reading
 and writing edit logs
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    [ https://issues.apache.org/jira/browse/HDFS-3077?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13422432#comment-13422432 ] 

Chao Shi commented on HDFS-3077:
--------------------------------

Hi Todd,

It's really cool that you're implementing a quorum-based journaling mechanism without extra dependencies. I just read the design doc and some pieces of your code. I feel that we can throw a special kind of fatal exception rather than a ordinary IOException, if any inconsistent states are found (e.g. a JN's epoch > QJM's epoch). A fatal exception means that QJM must immediately stop its work. This may be caused by mis-configuration or software bugs. Because that journal is so critical to HDFS clusters, we should try the best to detect any possible mistakes/bugs.

Besides that, I also suggest to store the last seen txid along with the epoch for each JN (maybe periodically), so that txid never decrease and we can have a double check for that. Because the algorithm to sync unclosed log section is complex, it would be nice to have such a simple approach to verify it.
                
> Quorum-based protocol for reading and writing edit logs
> -------------------------------------------------------
>
>                 Key: HDFS-3077
>                 URL: https://issues.apache.org/jira/browse/HDFS-3077
>             Project: Hadoop HDFS
>          Issue Type: New Feature
>          Components: ha, name-node
>            Reporter: Todd Lipcon
>            Assignee: Todd Lipcon
>             Fix For: QuorumJournalManager (HDFS-3077)
>
>         Attachments: hdfs-3077-partial.txt, hdfs-3077.txt, hdfs-3077.txt, hdfs-3077.txt, hdfs-3077.txt, hdfs-3077.txt, hdfs-3077.txt, hdfs-3077.txt, qjournal-design.pdf, qjournal-design.pdf
>
>
> Currently, one of the weak points of the HA design is that it relies on shared storage such as an NFS filer for the shared edit log. One alternative that has been proposed is to depend on BookKeeper, a ZooKeeper subproject which provides a highly available replicated edit log on commodity hardware. This JIRA is to implement another alternative, based on a quorum commit protocol, integrated more tightly in HDFS and with the requirements driven only by HDFS's needs rather than more generic use cases. More details to follow.

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