Joe Gregorio - ETech '07 Summary - Part 2 - MegaData:

"If you want to scale to the petabyte level, or the billion requests a day, you need to be:

Distributed

The data has to be distributed across multiple machines.

Joinless

No joins, and no referential integrity, at least at the data store level.

De-Normalized

No one said this explicily, but I presume there is a lot of de-normalization going on if you are avoiding joins.

Transcationless

No transactions

Those constraints represent something fundamentally different from a relational database."