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From "Liang Xie (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-5776) Support 'hedged' reads in DFSClient
Date Tue, 21 Jan 2014 06:29:22 GMT

    [ https://issues.apache.org/jira/browse/HDFS-5776?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13877263#comment-13877263
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Liang Xie commented on HDFS-5776:
---------------------------------

bq. When we get to the end of the pipeline here; i.e. all datanodes have been tried, what
happens?
In bestNode(), if we can not find any candidate, will throw new IOException("No live nodes
contain current block"), then chooseDataNode will catch it and retry within a calculated timeWindow
and dfsClient.getMaxBlockAcquireFailures limit.

bq. Say why it is ok to ignore the IOE at this point in the comment.
done

> 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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