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