[jira] Updated: (HADOOP-2559) DFS should place one replica per rack

["lohit vijayarenu (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/HADOOP-2559?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

lohit vijayarenu updated HADOOP-2559:
-------------------------------------

    Attachment: Trunk_Block_Report.png

Block Report across nodes after RandomWriter for Trunk

> 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.patch, HADOOP-2559-2.patch, Trunk_Block_Report.png
>
>
> 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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