Number of reducers in hadoop

Question

I was learning hadoop, I found number of reducers very confusing :

1) Number of reducers is same as number of partitions.

2) Number of reducers is 0.95 or 1.75 m

Answer 1

Your job may or may not need reducers, it depends on what are you trying to do. When there are multiple reducers, the map tasks partition their output, each creating one partition for each reduce task. There can be many keys (and their associated values) in each partition, but the records for any given key are all in a single partition. One rule of thumb is to aim for reducers that each run for five minutes or so, and which produce at least one HDFS block’s worth of output. Too many reducers and you end up with lots of small files.