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https://issues.apache.org/jira/browse/HADOOP-2559?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12579537#action_12579537
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Hairong Kuang commented on HADOOP-2559:
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+1 The patch looks good.
> 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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