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From "Arpit Agarwal (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-5776) Support 'hedged' reads in DFSClient
Date Tue, 28 Jan 2014 17:36:41 GMT

    [ https://issues.apache.org/jira/browse/HDFS-5776?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13884351#comment-13884351
] 

Arpit Agarwal commented on HDFS-5776:
-------------------------------------

[~stack] I am basically +1 on the v9 patch at this point but v10 is a step back. We need a
throttle on unbounded thread growth and threadpool size is the most trivial to add. We can
file a separate Jira to replace the thread pool limit with something more sophisticated e.g.
the client can keep a dynamic estimate of the 95th percentile latency and use that instead
of a fixed value from configuration.

Jing mentioned some issues that look fairly easy to address.

{quote}
In the old impl, the refetchToken/refetchEncryptionKey are shared by all nodes from chooseDataNode
once key/token exception happened. that means if the first node consumed this retry quota,
then if the second or third node hit the key/token exception, clearDataEncryptionKey/fetchBlockAt
opeerations will not be called, it's a little unfair
{quote}
[~xieliang007] That makes sense, thanks for the clarification.

> 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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