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From "Sanjay Radia (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-3077) Quorum-based protocol for reading and writing edit logs
Date Mon, 08 Oct 2012 17:06:03 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3077?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13471673#comment-13471673

Sanjay Radia commented on HDFS-3077:

bq. To be perfectly frank, I'm not interested in changing the design substantially ...

Todd the change is small and i am trying to help you here. Recall in HDFS-1073 you did not
want to use transaction ids or name the log files using transaction id range and you argued
against this for quite a while. As I predicted, txids have become a cornerstone of HA and
managing journals.

The only real change I have proposed to the protocol is to merge the first 2 operations.
* You have argued that prepare-recovery, using the epoch number from previous newEpoch, is
like multi-paxos - not sure if multi-paxos is warranted here. 
* The response of the newEpoch() is highest txId while response to PrepareRecovery is the
state of the highest segment, and optionally additional info if there was a previous recovery
in progress.
* BTW I am okay with the document describing the protocol by mapping it to both paxos and
zab for the reader.

> 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-test-merge.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, qjournal-design.pdf, qjournal-design.pdf, qjournal-design.pdf,
qjournal-design.pdf, qjournal-design.tex, qjournal-design.tex
> 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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