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From "Todd Lipcon (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-3077) Quorum-based protocol for reading and writing edit logs
Date Thu, 15 Mar 2012 22:18:39 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3077?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13230634#comment-13230634
] 

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

bq. The daemons can stop accepting writes when it realizes that active lock is no longer held
by the writer. Clearly an advantage of an active daemon compared to using passive storage.
Relying on ZK here is insufficient - the actual protocol itself needs fencing to guarantee
that a quorum of loggers have seen the "lost lock" before the new writer starts writing.

I agree with your later comments that rolling the edits is a helpful construct here, but you
need to also make sure there's consensus on the "active writer" when beginning a new log segment.

I'm about halfway done with a prototype implementation of this, I should have something to
show by middle of next week. At that point I'll also post a more thorough explanation of the
design.
                
> 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
>
> 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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