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From "Todd Lipcon (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-3077) Quorum-based protocol for reading and writing edit logs
Date Mon, 01 Oct 2012 23:39:12 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3077?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13467324#comment-13467324
] 

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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