Comment by hipadev23

Because that was the central point in the original whitepaper [1]: Hadoop is slow because it’s disk-only where Spark uses memory and caching to speed things up. I understand Spark isn’t 100% in-memory the way say Redis is, but it was still the major selling point vs. Hadoop.

https://people.csail.mit.edu/matei/papers/2010/hotcloud_spar...