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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 Sun, 07 Oct 2012 23:20:03 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3077?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13471352#comment-13471352

Sanjay Radia commented on HDFS-3077:

Wrt to epoch number in edits file: you raise the issues of consistency where a JM that writes
to local disk journal and also to QJM (QJM+localDisk journal); this is useful journal:
* the QJM is not replaceable by local disk journal if QJM is not available because the local
disk journal and qjm will not be consistent
* during recovery, the journal abstraction doesn't provide any mechanisms maintaining for
consistency across such "sister" journals.
Hence if we want to allow a QHM+localDisk journal we will need to revise the the journal manager
abstraction a little. BTW one could create a QJM+localDisk journal whose implementation does
exactly that with some hacks.

We may have to revise the journal abstraction at little to deal with the above situation (independent
of storing epoch in the first entry) since a QJM+localDisk journal is useful. I suspect that
the journal abstraction may need to have an operation called recover anyway. I am not sure
what change is needed exactly in the abstraction, but this particular issue can be addressed
in the next month or so.
> 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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