My understanding of hadoop and hdfs is that both input and ourput are to/from disk. Your MR jobs are spun up and pointed to hdfs urls to read input from and they write to hdfs when done. This means that when there is an error, the intermediate computations don't necessarily have to be redone. However there is a trade off.
Additionally, I believe that HDFS keeps 3 copies of the data around on 3 different nodes for redundancy. So there is the overhead of that network traffic.
It appears that spring double evaluates the expressions, so you can send a request param that is an EL expression that references values present in the server environment.
Can I ask that all product blogs provide a link to the actual site? Not everyone knows what layervault is and it would be nice to browse to the site without having to manipulate the address bar.