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Date: Mon, 17 Mar 2008 14:32:24 -0700 (PDT)
From: "lohit vijayarenu (JIRA)" <jira@apache.org>
To: core-dev@hadoop.apache.org
Subject: [jira] Commented: (HADOOP-2559) DFS should place one replica per
 rack
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    [ https://issues.apache.org/jira/browse/HADOOP-2559?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12579617#action_12579617 ] 

lohit vijayarenu commented on HADOOP-2559:
------------------------------------------

the failed test was due to timeout. I re-ran the test locally and it passed.

> DFS should place one replica per rack
> -------------------------------------
>
>                 Key: HADOOP-2559
>                 URL: https://issues.apache.org/jira/browse/HADOOP-2559
>             Project: Hadoop Core
>          Issue Type: Improvement
>          Components: dfs
>            Reporter: Runping Qi
>            Assignee: lohit vijayarenu
>         Attachments: HADOOP-2559-1-2.patch, HADOOP-2559-1-3.patch, HADOOP-2559-1-4.patch, HADOOP-2559-1.patch, HADOOP-2559-1.patch, HADOOP-2559-2.patch, Patch1_Block_Report.png.jpg, Patch1_Rack_Node_Mapping.jpg, Patch2 Block Report.jpg, Patch2_Rack_Node_Mapping.jpg, Trunk_Block_Report.png, Trunk_Rack_Node_Mapping.jpg
>
>
> Currently, when writing out a block, dfs will place one copy to a local data node, one copy to a rack local node
> and another one to a remote node. This leads to a number of undesired properties:
> 1. The block will be rack-local to two tacks instead of three, reducing the advantage of rack locality based scheduling by 1/3.
> 2. The Blocks of a file (especiallya  large file) are unevenly distributed over the nodes: One third will be on the local node, and two thirds on the nodes on the same rack. This may make some nodes full much faster than others, 
> increasing the need of rebalancing. Furthermore, this also make some nodes become "hot spots" if those big 
> files are popular and accessed by many applications.

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