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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 Tue, 09 Oct 2012 06:00:16 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3077?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13472161#comment-13472161
] 

Todd Lipcon commented on HDFS-3077:
-----------------------------------

bq. 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.

Wouldn't the same criticism hold for local storage? If you configure three local disk drives,
you get exactly the same behavior: if any drive throws an IOException, that drive is dropped
for further edits on that segment. If you have enabled the namedir restore configuration,
it will be retried on the next log roll. Otherwise, it is not retried until the NN is entirely
restarted.

The exact same is true of QJM, except that we always retry each JN on a log roll.

In both cases, on the read side, we look at all available directories (or JNs) and string
together a contiguous set of edits from whatever pieces exist.

Of course, in normal operation without failures, all directories (or JNs) will have a complete
history of edits.

If we want to improve this, let's treat it as an improvement after the merge. I'm happy to
work on it.
                
> 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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