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From "Liang Xie (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-5776) Support 'hedged' reads in DFSClient
Date Wed, 29 Jan 2014 07:14:09 GMT

    [ https://issues.apache.org/jira/browse/HDFS-5776?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13885091#comment-13885091
] 

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

bq. I think a better way is to add this check in chooseDataNode: if chooseDataNode finds that
this is for seeking the second DN (if ignored is not null), and it could not immediately/easily
find a DN, the chooseDataNode should skip retrying and we may want to fall back to the normal
read.
Yeh, sound reasonable. will look into it later once get chance.
P.S. i am taking a 8+ days long holiday(China Spring Festival) and probably can not reply
or make patch timely, sorry.

Happy Holiday to all guys, thanks for looking at this JIRA !!!

> 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-v10.txt, HDFS-5776-v2.txt, HDFS-5776-v3.txt, HDFS-5776-v4.txt,
HDFS-5776-v5.txt, HDFS-5776-v6.txt, HDFS-5776-v7.txt, HDFS-5776-v8.txt, HDFS-5776-v9.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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