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From "Joydeep Sen Sarma (JIRA)" <j...@apache.org>
Subject [jira] Commented: (MAPREDUCE-1800) using map output fetch failures to blacklist nodes is problematic
Date Thu, 20 May 2010 20:57:19 GMT

    [ https://issues.apache.org/jira/browse/MAPREDUCE-1800?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12869768#action_12869768
] 

Joydeep Sen Sarma commented on MAPREDUCE-1800:
----------------------------------------------

that makes a lot of sense. i think most of the failures we see are TCP errors - so yeah -
using just http application level error codes would help. Note that nic/port failure will
not produce http 5xx errors. so in order to cover that case (which may be common as well)
- we would have to incorporate network failures into fault diagnosis as well.

> 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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