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https://issues.apache.org/jira/browse/HDFS-3077?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13472099#comment-13472099
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Sanjay Radia commented on HDFS-3077:
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bq. Currently, we only run recovery on the highest txid segment at startup.
This means that every segment is stored on at least a quorum of nodes. But it
does not mean that previous segments get replicated to all available nodes.
This wasn't obvious from HDFS-3077 document and a limitation of HDFS-3077;
don't you agree? Segment holes is operationally messy when manual recovery is
necessary in the field.
Do the following two suggestions make sense?
# When a JN joins, it must have sync'ed all previous segments before accepting
new writes.
# At recovery, sync missing segments (due to 1, a JN may miss several segments
but the set of segments is all at the end - there cannot be holes.)
bq. If we wanted to improve this[deal with missing segments], however, ... If
we merged NewEpoch and PrepareRecovery, that wouldn't be possible.
Todd the way segments are playing out in our protocol is scaring me;
Zookeeper's ZAB avoids all this - they recover all previous transactions. It
seems that segments have complicated our protocol significantly.
With the additional subtleties you have pointed out I am worried only a few
will be able to maintain this code.
> 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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