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From "Jonathan Hsieh (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-7509) Enable RS to query a secondary datanode in parallel, if the primary takes too long
Date Thu, 02 Oct 2014 23:31:36 GMT

    [ https://issues.apache.org/jira/browse/HBASE-7509?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14157416#comment-14157416

Jonathan Hsieh commented on HBASE-7509:

I believe this can be closed. Reopen if I am wrong.

When using hadoop 2.4 we should just need to include the configs from the HDFS-5776 release
notes in the hbase-site.xml file: 
This feature is off by default. To enable this feature, set <code>dfs.client.hedged.read.threadpool.size</code>
to a positive number. The threadpool size is how many threads to dedicate to the running of
these 'hedged', concurrent reads in your client. 

Then set <code>dfs.client.hedged.read.threshold.millis</code> to the number of
milliseconds to wait before starting up a 'hedged' read. For example, if you set this property
to 10, then if a read has not returned within 10 milliseconds, we will start up a new read
against a different block replica. 

This feature emits new metrics: 

+ hedgedReadOps 
+ hedgeReadOpsWin -- how many times the hedged read 'beat' the original read 
+ hedgedReadOpsInCurThread -- how many times we went to do a hedged read but we had to run
it in the current thread because dfs.client.hedged.read.threadpool.size was at a maximum.

> Enable RS to query a secondary datanode in parallel, if the primary takes too long
> ----------------------------------------------------------------------------------
>                 Key: HBASE-7509
>                 URL: https://issues.apache.org/jira/browse/HBASE-7509
>             Project: HBase
>          Issue Type: Improvement
>            Reporter: Amitanand Aiyer
>            Assignee: Amitanand Aiyer
>            Priority: Critical
>         Attachments: quorumDiffs.tgz

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