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https://issues.apache.org/jira/browse/HDFS-3077?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13458196#comment-13458196
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Suresh Srinivas commented on HDFS-3077:
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Thanks for the detailed comments.
bq. given we already have the journal daemons, it's trivial to generate unique
increasing sequence IDs
But may still be unnecessary. May be during the code review I might find indeed
it is trivial.
bq. The third thing I'll mention is what I informally call the "two things"
problem
There is always "two things" problem :-)
In this case, you have leader/active(to loosely to put it) elected at zk and
then active has to establish epoch at znodes to become primary. Both of this
needs to be complete before an active becomes functional. Given the "two
things" that needs to happen, is a situation possible when one NN is active at
zk while not the primary at the journal nodes and the other NN is not active at
zk while is a primary at journal nodes. How will this be handled? Would this
require shutting down/fencing the other NN to prevent it from taking over as
primary at the journal nodes?
> 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.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
>
>
> 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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