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https://issues.apache.org/jira/browse/HDFS-12943?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16297939#comment-16297939
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Konstantin Shvachko commented on HDFS-12943:
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Chris, I do not have POC numbers. I believe [~csun] can elaborate on this.
I agree reads are blocking writes on NN.
Disallowing reads on active NN is an interesting twist. The design proposes a
new client-side config variable to enable reads from SBN. I think we can have
another one to disable reads from ANN:
* {{dfs.client.standby.reads.enabled = true}} - enables reads from standby
* {{dfs.client.active.reads.enabled = false}} - disables reads on active and
directs them exclusively to standby
> Consistent Reads from Standby Node
> ----------------------------------
>
> Key: HDFS-12943
> URL: https://issues.apache.org/jira/browse/HDFS-12943
> Project: Hadoop HDFS
> Issue Type: New Feature
> Components: hdfs
> Reporter: Konstantin Shvachko
> Attachments: ConsistentReadsFromStandbyNode.pdf
>
>
> StandbyNode in HDFS is a replica of the active NameNode. The states of the
> NameNodes are coordinated via the journal. It is natural to consider
> StandbyNode as a read-only replica. As with any replicated distributed system
> the problem of stale reads should be resolved. Our main goal is to provide
> reads from standby in a consistent way in order to enable a wide range of
> existing applications running on top of HDFS.
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