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courses:mapreduce-tutorial:step-24 [2012/01/27 22:08] straka |
courses:mapreduce-tutorial:step-24 [2012/01/27 22:20] straka |
===== Exercise ===== | ===== Exercise ===== |
Download the ''MapperOnlyHadoopJob.java'', compile it and run it using | Download the ''MapperOnlyHadoopJob.java'', compile it and run it using |
/net/projects/hadoop/bin/hadoop -r 0 MapperOnlyHadoopJob | /net/projects/hadoop/bin/hadoop -r 0 MapperOnlyHadoopJob.jar /home/straka/wiki/cs-text-small outdir |
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| Mind the ''-r 0'' switch -- specifying ''-r 0'' disable the reducer. If the switch ''-r 0'' was not given, one reducer of default type ''IdentityReducer'' would be used. The ''IdentityReducer'' outputs every (key, value) pair it is given. |
| * When using ''-r 0'', the job runs faster, as the mappers write the output directly to disk. Buth there are as many output files as mappers and the (key, value) pairs are stored in no special order. |
| * When not specifying ''-r 0'' (i.e., using ''-r 1'' with ''IdentityReducer''), the job produces the same (key, value) pairs. But this time they are in one output file, sorted according to the key. Of course, the job runs slower in this case. |