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From "Hadoop QA (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-4253) block replica reads get hot-spots due to NetworkTopology#pseudoSortByDistance
Date Tue, 04 Dec 2012 01:17:58 GMT

    [ https://issues.apache.org/jira/browse/HDFS-4253?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13509407#comment-13509407
] 

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/12555845/hdfs4253-1.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/3593//testReport/
Console output: https://builds.apache.org/job/PreCommit-HDFS-Build/3593//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.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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