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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 Wed, 14 Mar 2012 21:44:39 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3077?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13229661#comment-13229661
] 

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

>>We have a single writer, except for when we don't. During a failover, without a STONITH
capability,
>Without some sort of fencing, you're going to have to run agreement on every update. If
this is acceptable, you could have just made the namenode a thin RPC layer on top of zookeeper,
and you get fault tolerance for free.

Yea, as described in my comment this morning, there is a fencing operation built into the
logger daemons. Same as BK. So you only need a consensus about recovery. It's the same thing
as BK, similar to multi-paxos, etc -- the steady state is fast and you pay costs at leader
switchover.
                
> 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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