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From "Zhe Zhang (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-10702) Add a Client API and Proxy Provider to enable stale read from Standby
Date Fri, 17 Mar 2017 06:24:41 GMT

    [ https://issues.apache.org/jira/browse/HDFS-10702?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15929472#comment-15929472
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Zhe Zhang commented on HDFS-10702:
----------------------------------

Echoing [~mingma]'s concern on {{cpLock}}: I see a possible consistency issue. E.g. if a client
does {{getBlockLocations}} during a block report, SbNN could return an inconsistent result.

Also, how long does it usually take the SbNN to tail edit logs? For a client which has turned
on this stale read feature, if a read RPC happens to arrive at the SbNN during a edit log
tailing session, does it mean a long delay?

> 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: Improvement
>            Reporter: Jiayi Zhou
>            Assignee: Jiayi Zhou
>            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, 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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