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https://issues.apache.org/jira/browse/HADOOP-2559?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12575296#action_12575296
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lohit vijayarenu commented on HADOOP-2559:
------------------------------------------
Here are the test results
1. This test was 200 node random writer followed by just Scan.
{noformat}
Job Trunk Patch1 Patch2
RandWriter 460 471 520
Scan 718 583 601
MapTasks 16307 16307 16241
Data-Local Maps 15743 15741 15741
Rack-Local Maps 86 84 54
{noformat}
2. This test was 10 mappers on 200 node writing random text (total 2TB)
followed by 200 node Scan
{noformat}
Job Trunk Patch1 Patch2
RandWriter 13633 12923 12299
Scan 764 606 600
MapTasks 16517 16453 16538
Data-Local Maps 15276 15335 15695
Rack-Local Maps 433 363 121
{noformat}
> 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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