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From "Sanjay Radia (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-3077) Quorum-based protocol for reading and writing edit logs
Date Fri, 05 Oct 2012 01:59:52 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3077?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13469966#comment-13469966
] 

Sanjay Radia commented on HDFS-3077:
------------------------------------

Storing Epoch number in first transaction
Todd I think we should store the epoch number in the no-op transactions once consensus is
reached.
* You raised the objection that this breaks the Journal abstraction.  Think of this as an
"info-field" of the special  no-op transaction where the journal impl specific information
 is stored; this is fine since the roll operation automatically adds the non-op transaction
and as part of that the journal impl that is doing the roll can record information in the
info-field of the transaction. Indeed it should also store the writer's IP. Further, even
the local disk journal impl  should record the writer's IP.
* With respect to your comment on the copying journal from local disk of a NN to QJMs journal.
If you are doing this because the persistent state of the QJM is corrupt then it does not
work in the 3077's current impl because one has to also recreate the epoch file manually.
 
* As I mentioned earlier - Zookeeper also stores the epoch - the zxid contains the epoch number.
(BTW 3077's design is fairly close to ZAB and zookeeper's protocol - but more on this in another
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.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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