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From "Andy Isaacson (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (HDFS-4253) block replica reads get hot-spots due to NetworkTopology#pseudoSortByDistance
Date Mon, 03 Dec 2012 23:06:01 GMT

     [ https://issues.apache.org/jira/browse/HDFS-4253?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Andy Isaacson updated HDFS-4253:
--------------------------------

    Attachment: hdfs4253-1.txt

Uploading new patch using .equals rather than ==.
                
> 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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