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Date: Tue, 21 Jan 2014 05:53:22 +0000 (UTC)
From: "Liang Xie (JIRA)" <jira@apache.org>
To: hdfs-issues@hadoop.apache.org
Message-ID: <JIRA.12688930.1389757190823.38340.1390283602526@arcas>
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Subject: [jira] [Commented] (HDFS-5776) Support 'hedged' reads in DFSClient
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    [ https://issues.apache.org/jira/browse/HDFS-5776?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13877231#comment-13877231 ] 

Liang Xie commented on HDFS-5776:
---------------------------------

{quote}
getHedgedReadsThreadPool
Should be kept internal to DFSClient.
{quote}
changed from "public" to "protected", since we need it in DFSInputStream for a sanity check.

bq. So on a dfsclient instance, we can flip hedged reads on and off?
yes, will only take effect on that instance.

bq. Should fetchBlockByteRangeSpeculative be called fetchBlockByteRangeHedge or hedgedFetchBlockByteRange
ok, changed to "hedgedFetchBlockByteRange" :)

> Support 'hedged' reads in DFSClient
> -----------------------------------
>
>                 Key: HDFS-5776
>                 URL: https://issues.apache.org/jira/browse/HDFS-5776
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: hdfs-client
>    Affects Versions: 3.0.0
>            Reporter: Liang Xie
>            Assignee: Liang Xie
>         Attachments: HDFS-5776-v2.txt, HDFS-5776-v3.txt, HDFS-5776.txt
>
>
> This is a placeholder of hdfs related stuff backport from https://issues.apache.org/jira/browse/HBASE-7509
> The quorum read ability should be helpful especially to optimize read outliers
> we can utilize "dfs.dfsclient.quorum.read.threshold.millis" & "dfs.dfsclient.quorum.read.threadpool.size" to enable/disable the hedged read ability from client side(e.g. HBase), and by using DFSQuorumReadMetrics, we could export the interested metric valus into client system(e.g. HBase's regionserver metric).
> The core logic is in pread code path, we decide to goto the original fetchBlockByteRange or the new introduced fetchBlockByteRangeSpeculative per the above config items.


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