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https://issues.apache.org/jira/browse/HDFS-4253?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13532896#comment-13532896
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Hadoop QA commented on HDFS-4253:
---------------------------------

{color:green}+1 overall{color}.  Here are the results of testing the latest 
attachment 
  http://issues.apache.org/jira/secure/attachment/12561062/hdfs4253-2.txt
  against trunk revision .

    {color:green}+1 @author{color}.  The patch does not contain any @author 
tags.

    {color:green}+1 tests included{color}.  The patch appears to include 1 new 
or modified test files.

    {color:green}+1 javac{color}.  The applied patch does not increase the 
total number of javac compiler warnings.

    {color:green}+1 javadoc{color}.  The javadoc tool did not generate any 
warning messages.

    {color:green}+1 eclipse:eclipse{color}.  The patch built with 
eclipse:eclipse.

    {color:green}+1 findbugs{color}.  The patch does not introduce any new 
Findbugs (version 1.3.9) warnings.

    {color:green}+1 release audit{color}.  The applied patch does not increase 
the total number of release audit warnings.

    {color:green}+1 core tests{color}.  The patch passed unit tests in 
hadoop-common-project/hadoop-common hadoop-hdfs-project/hadoop-hdfs.

    {color:green}+1 contrib tests{color}.  The patch passed contrib unit tests.

Test results: 
https://builds.apache.org/job/PreCommit-HDFS-Build/3667//testReport/
Console output: https://builds.apache.org/job/PreCommit-HDFS-Build/3667//console

This message is automatically generated.
                
> block replica reads get hot-spots due to NetworkTopology#pseudoSortByDistance
> -----------------------------------------------------------------------------
>
>                 Key: HDFS-4253
>                 URL: https://issues.apache.org/jira/browse/HDFS-4253
>             Project: Hadoop HDFS
>          Issue Type: Bug
>    Affects Versions: 3.0.0, 2.0.2-alpha
>            Reporter: Andy Isaacson
>            Assignee: Andy Isaacson
>         Attachments: hdfs4253-1.txt, hdfs4253-2.txt, hdfs4253.txt
>
>
> When many nodes (10) read from the same block simultaneously, we get 
> asymmetric distribution of read load.  This can result in slow block reads 
> when one replica is serving most of the readers and the other replicas are 
> idle.  The busy DN bottlenecks on its network link.
> This is especially visible with large block sizes and high replica counts (I 
> reproduced the problem with {{-Ddfs.block.size=4294967296}} and replication 
> 5), but the same behavior happens on a small scale with normal-sized blocks 
> and replication=3.
> The root of the problem is in {{NetworkTopology#pseudoSortByDistance}} which 
> explicitly does not try to spread traffic among replicas in a given rack -- 
> it only randomizes usage for off-rack replicas.

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