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From "Kimoon Kim (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-331) map outputs should be written to a single output file with an index
Date Sat, 19 Aug 2006 01:13:15 GMT
    [ http://issues.apache.org/jira/browse/HADOOP-331?page=comments#action_12429168 ] 
            
Kimoon Kim commented on HADOOP-331:
-----------------------------------

Instead of one output, what about n outputs, where 1 <= n < # reducers? n can be configurable
and is set to a number that can avoid FS thrashing. Each output file contains a disjoint set
of partitions so that reducer in need of a partition will later access only one file. Increased
locality.

Given we are thinking of sorting map outputs,  this also allows us to avoid global sorting.
i.e. sorting happens on one output file at a time that has only small set of data.

> map outputs should be written to a single output file with an index
> -------------------------------------------------------------------
>
>                 Key: HADOOP-331
>                 URL: http://issues.apache.org/jira/browse/HADOOP-331
>             Project: Hadoop
>          Issue Type: Improvement
>          Components: mapred
>    Affects Versions: 0.3.2
>            Reporter: eric baldeschwieler
>         Assigned To: Yoram Arnon
>             Fix For: 0.6.0
>
>
> The current strategy of writing a file per target map is consuming a lot of unused buffer
space (causing out of memory crashes) and puts a lot of burden on the FS (many opens, inodes
used, etc).  
> I propose that we write a single file containing all output and also write an index file
IDing which byte range in the file goes to each reduce.  This will remove the issue of buffer
waste, address scaling issues with number of open files and generally set us up better for
scaling.  It will also have advantages with very small inputs, since the buffer cache will
reduce the number of seeks needed and the data serving node can open a single file and just
keep it open rather than needing to do directory and open ops on every request.
> The only issue I see is that in cases where the task output is substantiallyu larger
than its input, we may need to spill multiple times.  In this case, we can do a merge after
all spills are complete (or during the final spill).

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