Hadoop Map/Reduce vs built-In Map/Reduce

Question

What are the key differences to do map/reduce work on MongoDB using Hadoop map/reduce vs built-in map/reduce of Mongo?

When do I pick which map/reduce engine? what a

Answer 1

I don't have a lot of experience with Hadoop MR, but my impression is that it only works on HDFS, so you would have to duplicate all of your Mongo data in HDFS. If you are willing to duplicate all of your data, I would guess Hadoop MR is much faster and more robust than Mongo MR.