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From "Todd Lipcon (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-3077) Quorum-based protocol for reading and writing edit logs
Date Tue, 03 Apr 2012 19:56:23 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3077?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13245656#comment-13245656

Todd Lipcon commented on HDFS-3077:

bq. So currently, state is epoch number is 2 on all the journals and J1, J2 and J3 are at
153. We have a problem since it is not possible to distinguish between log entries in J1 vs
J2 and J3.

Hey Hari. Thanks for taking a look in such good detail.

I think the doc is currently unclear about the proposed solution described in 2.5.6 -- the
idea is not to use just the "lastPromisedEpoch" here to distinguish the JNs, but rather to
attach the epoch number to each log segment, based on the epoch in which that segment was
started. So, even though in your scenario NN1 sets J1.lastPromisedEpoch=2, the log segment
will retain e=1. Once a segment's epoch is set, it is never changed (unless the segment is
removed by a synchronization)

Does that make sense? If so I will try to clarify the document.
> 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
>         Attachments: hdfs-3077-partial.txt, 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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