Depending on how people use spark, this might not be the most prudent advice. For instance, if they tend to load a dataset from HDFS once and then do a bunch of iterative and shuffle-heavy jobs on it, it would be better to just put all of the spark nodes co-resident on one rack. Many of the clusters I've seen so far actually do this - they just have a single rack with a bunch of beefy Spark nodes for this reason. It might be good to either remove this as the "default" recommendation, or give some sense of when it's better to put all spark nodes on the same rack.