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https://issues.apache.org/jira/browse/HADOOP-692?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12463431
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Hairong Kuang commented on HADOOP-692:
--------------------------------------

Although the 2nd replica is placed off-rack, the 3rd replica is placed on the 
same rack as the first replica. Besides chooseTarget returns a sorted list of 
the choosen targets so that they form a shortest path that starts from the 
writer and traverses all the targets. So in the returned list, the 1st element 
is the local node, 2nd is the same-rack node, and 3rd is the off-rack node.

> Rack-aware Replica Placement
> ----------------------------
>
>                 Key: HADOOP-692
>                 URL: https://issues.apache.org/jira/browse/HADOOP-692
>             Project: Hadoop
>          Issue Type: Improvement
>          Components: dfs
>    Affects Versions: 0.8.0
>            Reporter: Hairong Kuang
>         Assigned To: Hairong Kuang
>         Attachments: rack.patch, Rack_aware_HDFS_proposal.pdf
>
>
> This issue assumes that HDFS runs on a cluster of computers that spread 
> across many racks. Communication between two nodes on different racks needs 
> to go through switches. Bandwidth in/out of a rack may be less than the total 
> bandwidth of machines in the rack. The purpose of rack-aware replica 
> placement is to improve data reliability, availability, and network bandwidth 
> utilization. The basic idea is that each data node determines to which rack 
> it belongs at the startup time and notifies the name node of the rack id upon 
> registration. The name node maintains a rackid-to-datanode map and tries to 
> place replicas across racks.

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