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From "Suresh Srinivas (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-3077) Quorum-based protocol for reading and writing edit logs
Date Tue, 18 Sep 2012 21:31:12 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3077?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13458196#comment-13458196
] 

Suresh Srinivas commented on HDFS-3077:
---------------------------------------

Thanks for the detailed comments.
bq. given we already have the journal daemons, it's trivial to generate unique increasing
sequence IDs 
But may still be unnecessary. May be during the code review I might find indeed it is trivial.

bq. The third thing I'll mention is what I informally call the "two things" problem

There is always "two things" problem :-)

In this case, you have leader/active(to loosely to put it) elected at zk and then active has
to establish epoch at znodes to become primary. Both of this needs to be complete before an
active becomes functional. Given the "two things" that needs to happen, is a situation possible
when one NN is active at zk while not the primary at the journal nodes and the other NN is
not active at zk while is a primary at journal nodes. How will this be handled? Would this
require shutting down/fencing the other NN to prevent it from taking over as primary at the
journal nodes?

                
> 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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