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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 Fri, 05 Oct 2012 21:30:07 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3077?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13470680#comment-13470680
] 

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

bq. I'm not interested in changing the design substantially at this point without a good reason.
I've put several weeks into testing this design...

Todd, several people have put in a bunch of time in this as well, not just in this jira, but
also related HA jiras and HDFS-3092 which this jira incorporates. 

Just reviewing this design has required catching up with subtleties of Paxos and ZAB. So the
comments made here are not frivolous, but come with quite a bit of investment of time. The
design started saying it is ZAB like, then added paxos and since then has gone back and forth.
The interest in getting this right is to ensure a critical component of HDFS is right.

The reason to do this is to make sure not only the design is correct, but is simple and completely
documented. This is more important given the complexity of this solution and the need for
other HDFS contributors to understand so it can be maintained. This has been brought up Konstantin
also in the mailing list.

I know you have spent a lot of time on this. I am okay if you do not want to make further
changes. But I think some of the changes that Sanjay is proposing is worth considering, if
it simplifies the design. In general, a reviewer's feedback for a jira is not just to show
design bugs, but also to provide feedback on other things, such as maintainability, simplicity
etc. 

If you do not have cycles, it is understandable. Someone else can pick up the work that comes
out of these discussions. 
                
> 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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