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Date: Fri, 30 Nov 2012 21:27:58 +0000 (UTC)
From: "Andy Isaacson (JIRA)" <jira@apache.org>
To: hdfs-dev@hadoop.apache.org
Message-ID: <1822213320.47113.1354310878849.JavaMail.jiratomcat@arcas>
Subject: [jira] [Created] (HDFS-4253) block replica reads get hot-spots due
 to NetworkTopology#pseudoSortByDistance
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Andy Isaacson created HDFS-4253:
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             Summary: 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: 2.0.2-alpha, 3.0.0
            Reporter: Andy Isaacson
            Assignee: Andy Isaacson


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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