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Date: Tue, 28 Apr 2009 18:47:30 -0700 (PDT)
From: "Konstantin Shvachko (JIRA)" <jira@apache.org>
To: core-dev@hadoop.apache.org
Subject: [jira] Updated: (HADOOP-5734) HDFS architecture documentation
 describes outdated placement policy
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     [ https://issues.apache.org/jira/browse/HADOOP-5734?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Konstantin Shvachko updated HADOOP-5734:
----------------------------------------

    Component/s: dfs

> HDFS architecture documentation describes outdated placement policy
> -------------------------------------------------------------------
>
>                 Key: HADOOP-5734
>                 URL: https://issues.apache.org/jira/browse/HADOOP-5734
>             Project: Hadoop Core
>          Issue Type: Bug
>          Components: dfs, documentation
>    Affects Versions: 0.20.0
>            Reporter: Konstantin Boudnik
>            Assignee: Konstantin Boudnik
>            Priority: Minor
>             Fix For: 0.21.0
>
>         Attachments: HADOOP-5734.patch
>
>          Time Spent: 2h
>  Remaining Estimate: 0h
>
> The "Replica Placement: The First Baby Steps" section of HDFS architecture document states:
> "...
> For the common case, when the replication factor is three, HDFS's placement policy is to put one replica on one node in the local rack, another on a different node in the local rack, and the last on a different node in a different rack. This policy cuts the inter-rack write traffic which generally improves write performance.
> ..."
> However, according to the ReplicationTargetChooser.chooseTarger()'s code the actual logic is to put the second replica on a different rack as well as the third replica. So you have two replicas located on a different nodes of remote rack and one (initial replica) on the local rack's node. Thus, the sentence should say something like this:
> "For the common case, when the replication factor is three, HDFS's placement policy is to put one replica on one node in the local rack, another on a node in a different (remote) rack, and the last on a different node in the same remote rack. This policy cuts the inter-rack write traffic which generally improves write performance."

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