When everything doesn’t fit onto a computer, you have to be able to migrate data to multiple nodes. You need some sort of scaling solution there… MapReduce works as a solution when your queries are operating over a lot of data; Google sizes of data. Few companies have Google-sized datasets though. The average sites you see, they’re 10-20 gigs of data.
Users shouldn’t need to put that data onto multiple machines anyway. In fact, I don’t think we need a multi-machine solution for the common case at all. We need software that can scale up with today’s hardware. 37signals likes to run boxes with half a terabyte of RAM. Are we there yet with MySQL and InnoDB? No. Postgres? No. Anything open-source? Not that I know of. We’ve got database software that can only do a fraction of what it should be able to on that size of server.
I think we have to be clear about the use case for a solution that partitions data across multiple machines. It isn’t 20GB of data, and in my opinion it shouldn’t even be half a terabyte. I think that in the ideal world, we should be thinking about that for terabytes and larger – and in a few years, single-server datasets should be even larger.
I say should because today’s database software obviously has a lot of catching up to do.