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https://issues.apache.org/jira/browse/MAPREDUCE-2198?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12934939#action_12934939
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Arun C Murthy commented on MAPREDUCE-2198:
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bq. Also, as Joydeep pointed out there are several issues with unassigned slots 
in TTs...

You also have to worry about piggy-backing of task-cleanup tasks done by the 
JT...

> Allow FairScheduler to control the number of slots on each TaskTracker
> ----------------------------------------------------------------------
>
>                 Key: MAPREDUCE-2198
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-2198
>             Project: Hadoop Map/Reduce
>          Issue Type: New Feature
>          Components: contrib/fair-share
>    Affects Versions: 0.22.0
>            Reporter: Scott Chen
>            Assignee: Scott Chen
>             Fix For: 0.22.0
>
>
> We can set the number of slots on the TaskTracker to be high and let 
> FairScheduler handles the slots.
> This approach allows us to change the number of slots on each node 
> dynamically.
> The administrator can change the number of slots with a CLI tool.
> One use case of this is for upgrading the MapReduce.
> Instead of restarting the cluster, we can run the new MapReduce on the same 
> cluster.
> And use the CLI tool to gradually migrate the slots.
> This way we don't lost the progress fo the jobs that's already executed.

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