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From "Ming Ma (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-10702) Add a Client API and Proxy Provider to enable stale read from Standby
Date Wed, 13 Sep 2017 05:10:02 GMT

    [ https://issues.apache.org/jira/browse/HDFS-10702?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16164158#comment-16164158
] 

Ming Ma commented on HDFS-10702:
--------------------------------

[~csun], the long checkpoint duration could cause the following issue:

* Checkpointer holding {{cpLock}} takes a long time to finish for a large namespace.
* edit log tailer blocked by {{cpLock}} can't update namespace. Thus the namespace becomes
stale.
* An application deletes a file. StandbyNN receives incremental block report indicating the
blocks have been removed and update its blockmap.
* StandbyNN's stale namespace still has the file but without block locations. 

> Add a Client API and Proxy Provider to enable stale read from Standby
> ---------------------------------------------------------------------
>
>                 Key: HDFS-10702
>                 URL: https://issues.apache.org/jira/browse/HDFS-10702
>             Project: Hadoop HDFS
>          Issue Type: New Feature
>            Reporter: Jiayi Zhou
>            Assignee: Sean Mackrory
>            Priority: Minor
>         Attachments: HDFS-10702.001.patch, HDFS-10702.002.patch, HDFS-10702.003.patch,
HDFS-10702.004.patch, HDFS-10702.005.patch, HDFS-10702.006.patch, HDFS-10702.007.patch, HDFS-10702.008.patch,
StaleReadfromStandbyNN.pdf
>
>
> Currently, clients must always talk to the active NameNode when performing any metadata
operation, which means active NameNode could be a bottleneck for scalability. One way to solve
this problem is to send read-only operations to Standby NameNode. The disadvantage is that
it might be a stale read. 
> Here, I'm thinking of adding a Client API to enable/disable stale read from Standby which
gives Client the power to set the staleness restriction.



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