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From "Chen Liang (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-12943) Consistent Reads from Standby Node
Date Mon, 12 Nov 2018 18:32:01 GMT

    [ https://issues.apache.org/jira/browse/HDFS-12943?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16684222#comment-16684222
] 

Chen Liang commented on HDFS-12943:
-----------------------------------

[~xiangheng] thanks for trying Observer read! What was the full command you ran? It should
be something like {{hdfs haadmin -transitionToObserver <nnID>}} where nnID is the ID
of the name node that you want to transition to Observer. You can run {{hdfs haadmin -getAllServiceState}}
to list all the valid nnIDs in the cluster.

> 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
>            Priority: Major
>         Attachments: ConsistentReadsFromStandbyNode.pdf, ConsistentReadsFromStandbyNode.pdf,
TestPlan-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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