Hi, @rxin , I'm also a bit confused here, I think the name of the argument is a bit confusing

https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/rdd/RDD.scala#L357

The above line contains a multiplier to ensure that the sampling can return enough sample points in most of cases..(I think so), so the fraction value can actually be larger than 1

also, this value actually determines the mean value of Poisson/Bernoulli distribution 

https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/rdd/RDD.scala#L314