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From "dhruba borthakur (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-692) Rack-aware Replica Placement
Date Thu, 09 Nov 2006 23:10:38 GMT
    [ http://issues.apache.org/jira/browse/HADOOP-692?page=comments#action_12448606 ] 
            
dhruba borthakur commented on HADOOP-692:
-----------------------------------------

I have a two questions: 

1. Is the rackId associated with a datanode or is it associated with the data directories
in a datanode? 
2. Suppose that all three replicas was allocated in the same rack because there weren't any
space available on other racks. Now, if more racks are added to the system, will HDFS automatically
rebalance the replicas to conform to your rack-aware logic?


> Rack-aware Replica Placement
> ----------------------------
>
>                 Key: HADOOP-692
>                 URL: http://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
>             Fix For: 0.9.0
>
>
> 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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