Tao Jie commented on HDFS-13418:

[~linyiqun] Thank you for your comment. I feel a little tricky that 
{{net.topology.impl}} is configured but does not work when 
{{dfs.use.dfs.network.topology}} is true.
 1, Can we just remove 
{{net.topology.impl=org.apache.hadoop.net.NetworkTopology}} in 
core-default.xml? Since we have default impl in code.
 2, Can we set {{net.topology.impl=org.apache.hadoop.net.DFSNetworkTopology}} 
in hdfs-default.xml? So it would cover the value set in core-site.default when 
we use HDFS.
 3, If we want to add a new NetworkTopology impl and work with 
{{DFSNetworkTopology}}, we may define the new NetworkTopology extends 
{{DFSNetworkTopology}}. But it does not work since {{DFSNetworkTopology}} is 
hardcoded here once {{dfs.use.dfs.network.topology}} is true. Can we use 
reflection to instantiate the NetworkTopology impl when 
{{dfs.use.dfs.network.topology}} is set true?

>  NetworkTopology should be configurable when enable DFSNetworkTopology
> ----------------------------------------------------------------------
>                 Key: HDFS-13418
>                 URL: https://issues.apache.org/jira/browse/HDFS-13418
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>    Affects Versions: 3.0.1
>            Reporter: Tao Jie
>            Assignee: Tao Jie
>            Priority: Major
> In HDFS-11530 we introduce DFSNetworkTopology and in HDFS-11998 we set 
> DFSNetworkTopology as the default implementation.
> We still have {{net.topology.impl=org.apache.hadoop.net.NetworkTopology}} in 
> core-site.default. Actually this property does not effect once 
> {{dfs.use.dfs.network.topology}} is true. 
> in {{DatanodeManager}},networkTopology is initialized as 
> {code}
>     if (useDfsNetworkTopology) {
>       networktopology = DFSNetworkTopology.getInstance(conf);
>     } else {
>       networktopology = NetworkTopology.getInstance(conf);
>     }
> {code}
> I think we should still make the NetworkTopology  configurable rather than 
> hard code the implementation since we may need another NetworkTopology impl.
> I am not sure if there is other consideration. Any thought? [~vagarychen] 
> [~linyiqun]

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