Designing Solutions on a Converged Infrastructure

I've been doing a lot of thinking (no answers today, just thoughts) about the solutions and applications running on top of converged infrastructure stacks.  Today, we have a few scenarios to consider that each require increasing levels of design complexity:

1. A single tenant with a single application (one big VDI farm or maybe Cisco Hosted Collaboration Solution) 
2. A single tenant with multiple applications (a VDI farm as well as an SAP instance) 
3. Multiple tenants running the same application (vCloud Director giving out IaaS resources)
4. Multiple tenants running multiple applications (a service provider hosting vCloud Director as well as Cisco Unified Communications)

Single Tenant with a Single Application - This one is pretty easy to digest; the entire CI (converged infrastructure) can be tweaked and modified to support a single application.  This makes the technical folks happy because they get to play with the "nerd knobs" and do some tuning.  The downside is that unless the application is a really big application, the CI may not be fully utilized or you may have multiple CI's in the data center.  You've just taken the server sprawl problem of a few years ago (single application, single server) and made it worse.

Single Tenant with Multiple Applications - In this scenario we just need to make sure our applications "play well with others".  As long as the CI can support the design, you are just adding more "blocks" at the application level.  This design usually requires a certain level of interop testing to make sure one application doesn't starve the other for resources.

Multiple Tenants Running the Same Application - The classic example here is vCloud Director.  As many are finding out, defining a work area for multiple tenants while at the same time creating a secure environment can be challenging.  When hosting multiple customers or tenants, it's all about the isolation.

Multiple Tenants Running Multiple Applications - This is where the fun begins!  You now have to balance the isolation aspects of multiple tenants while also providing a level of service across all the applications, all in a secure and stable environment.  For CI's to really succeed, this is where our design focus as architects needs to be in the future.

A Note on Multiple Tenants: As brought up by Christian Reilly on a recent episode of the Cloudcast, where do you define Multi-Tenants?  Are you talking just at the hardware layer?  Can you share a database and host multiple instances (a very common practice) or do you need separate databases servers as well?  Things to think about...