[jira] Commented: (HADOOP-1985) Abstract node to switch mapping into a topology service class used by namenode and jobtracker

["Runping Qi (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HADOOP-1985?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12533350 ] 

Runping Qi commented on HADOOP-1985:
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yes, DNS name (hostname) to switch id mapping should be managed just like the hostname to IP mapping. 
The info should be available to the DFS  namenode, datanodes and applications in the same way. 
Job tracker uses this info for task assignment. In general, DFS client should also use this info to decide where to fetch a needed block.


> Abstract node to switch mapping into a topology service class used by namenode and jobtracker
> ---------------------------------------------------------------------------------------------
>
>                 Key: HADOOP-1985
>                 URL: https://issues.apache.org/jira/browse/HADOOP-1985
>             Project: Hadoop
>          Issue Type: New Feature
>            Reporter: eric baldeschwieler
>
> In order to implement switch locality in MapReduce, we need to have switch location in both the namenode and job tracker.  Currently the namenode asks the data nodes for this info and they run a local script to answer this question.  In our environment and others that I know of there is no reason to push this to each node.  It is easier to maintain a centralized script that maps node DNS names to switch strings.
> I propose that we build a new class that caches known DNS name to switch mappings and invokes a loadable class or a configurable system call to resolve unknown DNS to switch mappings.  We can then add this to the namenode to support the current block to switch mapping needs and simplify the data nodes.  We can also add this same callout to the job tracker and then implement rack locality logic there without needing to chane the filesystem API or the split planning API.
> Not only is this the least intrusive path to building racklocal MR I can ID, it is also future compatible to future infrastructures that may derive topology on the fly, etc, etc...

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