Again ... this is more an indictment on the poor IO performance of Amazon EBS vs. MongoDB as a solution. MongoDB can scale both vertically and horizontally, but as with anything you scale on Amazon infrastructure, you are going to have to really think through strategies for dealing with the unpredictable performance of EBS. There are blog posts galore addressing this fact.
I often think MongoDB has suffered more as a young technology because of the proliferation of the AWS Cloud and the expectations of EBS performance.
I often think MongoDB has suffered more as a young technology because of the proliferation of the AWS Cloud and the expectations of EBS performance.