map outputs should be written to a single output file with an index
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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
Assigned to: 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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