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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 23:26:42 GMT
    [ http://issues.apache.org/jira/browse/HADOOP-692?page=comments#action_12448893 ] 
Doug Cutting commented on HADOOP-692:

How about a topology interface?

public interface NetworkTopology {
  HubDistance[] getHubDistances(String host)

public interface HubDistance {
  public String getHubName();
  public int getHops();

The namenode can use this to refresh things dynamically, or it might be read statically from
a config file, depending on the implementation.  The implementation class can be specified
in the config.  The default could just chop up hostnames, so that foo.bar.co.uk yeilds hops
<foo.bar.co.uk,0>, <bar.co.uk,2>, <co.uk,3>, <uk,4>.  Someone could
implement this with DNS too.

To compute a distance, you find the common hub between two nodes and sum the hops.  To find
nearby nodes, look for nodes sharing a nearby hub.  Etc.

> 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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