Vadim and I have just published a new technical white paper. It shows how Percona Server with XtraDB can make large-scale multi-tenant databases easier to build with MySQL. Our experiences working with SaaS and shared-hosting companies influenced the features we included in Percona Server and XtraDB, and I think this is the best explanation of what levers are available and how to use them.
Percona Server with XtraDB for Software-as-a-Service Application Databases is posted on the white-paper section of our website. No registration is required; it is just a PDF, freely downloadable with no marketing follow-up. Many thanks to Mark Callaghan and Dimitri Kravtchuk, who reviewed and gave valuable suggestions that made the white paper better.