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From "Doug Cutting (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-692) Rack-aware Replica Placement
Date Fri, 10 Nov 2006 21:39:38 GMT
    [ http://issues.apache.org/jira/browse/HADOOP-692?page=comments#action_12448867 ] 
            
Doug Cutting commented on HADOOP-692:
-------------------------------------

I think that at this point what we want is a declarative description of the cluster topology,
provided by the administrator, rather than a distance function.  For example, we could extend
the format of the slaves file to list all of the network levels that a node lives at:

<hostname> <rackswitchname> <clusterswitchname> <datacentername>,
etc.

This can be used not just to compute the distance between any two nodes, but also to quickly
list nodes that are close to a node.  The latter is hard with a distance function without
pre-computing all inter-node distances.  With 10k nodes, that's 100M distances, which is expensive.

> 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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