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From "Chao Shi (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-3077) Quorum-based protocol for reading and writing edit logs
Date Fri, 27 Jul 2012 01:37:37 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3077?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13423621#comment-13423621
] 

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

bq. 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.

I think it over again today and find my example "JN's epoch > QJM's epoch" may be wrong,
because it is the normal case that an old writer is fenced. When a InvariantViolatedException
is thrown, we expect that someone on call should be paged and go to check the cluster immediately.
So false alarming would be annoying anyway.
                
> 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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