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From "paul sutter (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-331) map outputs should be written to a single output file with an index
Date Thu, 29 Jun 2006 17:32:31 GMT
    [ http://issues.apache.org/jira/browse/HADOOP-331?page=comments#action_12418501 ] 

paul sutter commented on HADOOP-331:

one more suggestion (plea): could the map-output-spill-extraction take place on the mapper
side (perhaps in the http server), such that the reducer be able to retreive fully-formed
map outputs as currently?

incidently, we have individual map inputs that are 5-10GB in size, which are compressed so
they cant be split, which would yield potentially 50-100 spills per mapper. it would be nice
if the reducer (or whatever process is fetching the files) didnt have to do that reassembly,
but rather that it happened on the mapper side.

> 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
>         Type: Improvement

>   Components: mapred
>     Versions: 0.3.2
>     Reporter: eric baldeschwieler
>     Assignee: Yoram Arnon
>      Fix For: 0.5.0

> 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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