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https://issues.apache.org/jira/browse/HDFS-12943?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16300661#comment-16300661
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Konstantin Shvachko commented on HDFS-12943:
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??what's the function of the Observer Nodes??
Good question. The design doc says that Observer Node is an SBN that does not
do checkpoints. Checkpointing degrades performance of SBN, we wont be able to
read from it when it's busy. So it's more like a term to distinguish the node
which is dedicated for reading - the read-only SBN. Regular SBN is also needed
though if we want checkpointing and HA on the cluster, which I do. In the "Note
on HA" we talk about some failover scenarios, that reading from ObserverNode
elevates it role on the cluster so that you may need to run multiple of them to
sustain the response rate in case of failure.
> 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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