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From "Todd Lipcon (JIRA)" <j...@apache.org>
Subject [jira] Commented: (MAPREDUCE-1800) using map output fetch failures to blacklist nodes is problematic
Date Wed, 19 May 2010 18:20:54 GMT

    [ https://issues.apache.org/jira/browse/MAPREDUCE-1800?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12869247#action_12869247
] 

Todd Lipcon commented on MAPREDUCE-1800:
----------------------------------------

Hey Joydeep. Do you often have cases where sets of TT nodes can't talk to each other but both
sides can still talk to the JT? This is interesting, as it seems like an unusual network architecture.

> using map output fetch failures to blacklist nodes is problematic
> -----------------------------------------------------------------
>
>                 Key: MAPREDUCE-1800
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-1800
>             Project: Hadoop Map/Reduce
>          Issue Type: Bug
>            Reporter: Joydeep Sen Sarma
>
> If a mapper and a reducer cannot communicate, then either party could be at fault. The
current hadoop protocol allows reducers to declare nodes running the mapper as being at fault.
When sufficient number of reducers do so - then the map node can be blacklisted. 
> In cases where networking problems cause substantial degradation in communication across
sets of nodes - then large number of nodes can become blacklisted as a result of this protocol.
The blacklisting is often wrong (reducers on the smaller side of the network partition can
collectively cause nodes on the larger network partitioned to be blacklisted) and counterproductive
(rerunning maps puts further load on the (already) maxed out network links).
> We should revisit how we can better identify nodes with genuine network problems (and
what role, if any, map-output fetch failures have in this).

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