Database parallelism choices greatly impact scalability
Sam Madden in The Database Column - Database parallelism choices greatly impact scalability:
"Because shared nothing [as in IBM DB2] does not typically have nearly as severe bus or resource contention as shared-memory [MySQL] or shared-disk [Oracle RAC] machines, shared nothing can be made to scale to hundreds or even thousands of machines. Because of this, it is generally regarded as the best-scaling architecture. "