Workloads have to use the network to do the following : 1. Shift data onto HDFS (obviously), this is rare though 2. Write results after reduce (or you can't re

Workloads have to use the network to do the following : 1. Shift data onto HDFS (obviously), this is rare though 2. Write results after reduce (or you can't read them) when you are doing ETL this can be significant; you are sorting out an image for a production system and this is non-trivial abouts of data (100's GB at least) 3. Shuffle; this is the worry (for me) MR jobs can shuffle a lot of data around pre reduce.