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https://issues.apache.org/jira/browse/HDFS-3077?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13467324#comment-13467324
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Todd Lipcon commented on HDFS-3077:
-----------------------------------
{quote}
Persisting epoc number and recovery state.
You store this in the local disk separate from the segment. The epoc number
(and also the ip address of the writer) should be stored in the segment's first
record. I believe ZK's txId contains the epoch number; I am not suggesting
extending the txId, but putting the Epoch number in the first transaction is
roughly equivalent. This avoids the extra local disk data structures, is useful
for debugging in the field.
{quote}
This would involve a new edit log format for the JournalNode's edit storage,
distinct from the edit log format used on the local disk. I wanted to avoid
that, so that you could freely copy edit log files to/from JournalNodes to
old-style shared storage, run existing tools like the offline edits viewer,
etc. We can't simply put it in the first transaction, because that would imply
that the JournalManager sets the transaction's contents, breaking the
abstraction that this is just edits storage (per my earlier comment)
> 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.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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