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https://issues.apache.org/jira/browse/HDFS-6469?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14049227#comment-14049227
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Sanjay Radia commented on HDFS-6469:
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Wrt to double journaling
bq. If I follow your logic correctly QJM being Paxos-based uses a journal by
itself, so we are not increasing journaling here. When you look at the bigger
picture we see more journals around. HBase uses WAL along with NN edits, which
by itself persisted in ext4 a journaling file system.
Konstatine, Todd's point is not that there are multiple journal in the system
but that every update operation of NN, will result in an entry in two journals:
HDFS's edit log and the journal used by the consensusNode paxos protocol.
Your example of HBases log and NN log is not a good comparison: every write to
the HBase WAL does NOT result in a HDFS editlog entry - an entry is made in the
HDFS editlog ONLY when the WAL crosses a block boundary.
> Coordinated replication of the namespace using ConsensusNode
> ------------------------------------------------------------
>
> Key: HDFS-6469
> URL: https://issues.apache.org/jira/browse/HDFS-6469
> Project: Hadoop HDFS
> Issue Type: New Feature
> Components: namenode
> Affects Versions: 3.0.0
> Reporter: Konstantin Shvachko
> Assignee: Konstantin Shvachko
> Attachments: CNodeDesign.pdf
>
>
> This is a proposal to introduce ConsensusNode - an evolution of the NameNode,
> which enables replication of the namespace on multiple nodes of an HDFS
> cluster by means of a Coordination Engine.
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