From Scale Up vs. Scale Out...to Scale Down

Those of you that have been following me on twitter and on my blog know that I have been very focused on studying and monitoring the latest trends regarding which hardware platforms virtualization users are using for their infrastructures. This includes multiple points of view such as simple sizing rules of thumb, potential reference architectures and scale up vs. scale out strategies. I'd like to spend the next few minutes talking about what's going on lately in this respect, specifically in light of the latest (and future) hardware improvements we have seen or that we will see in the next few months. I am doing this because I have a very weird feeling about what's going on. Bear with me.

When I started working with VMware software back in 2001, the only value proposition that we could imagine out of the thing was the so-called server consolidation: in essence the process of consolidating many virtual instances - aka partitions or guests - onto a fewer number of physical servers. To make a long story short, down the road we have realized that the value proposition was way more than just server consolidation as a mean to reduce the costs of operation. It suddenly became pretty evident that there were many more advantages to that which may include things like easier high-availability for applications, easier Disaster Recovery scenarios, faster time-to-market for business applications, and many more. Server consolidation was, at that point, just one of the many value items we know today.

Right now my feeling is that the advantage of stuffing more and more OS instances on as few physical systems as possible is not even considered an advantage any more these days. To put it another way, it is still considered an advantage, but only to a certain extent. In fact, if consolidating more instances on fewer hardware pieces was still one of the strategic objectives of a virtualization process, what you would have seen was a progression in terms of the ratio # of OS instances / physical system. Something like this:

• 4-Socket single-core x86-based server with n GB of memory could support 10 VMs
• 4-Socket dual-core x86-based server with n*2 GB of memory could support 20 VMs
• 4-Socket quad-core x86-based server with n*4 GB of memory could support 40 VMs

The numbers above are just examples, and are only used to outline the mathematic progression I was mentioning. The high level idea behind it is that, the more powerful the systems become, the more OS instances you could consolidate onto them. Once you have strategically chosen a given hardware platform (whose main characteristic is expressed in # of CPUs it is capable to support) you will see higher consolidation ratios as the CPUs become more powerful (typically via doubling the number of cores from one generation to the other). Put into a more mathematical language, the constant here should be the number of CPUs (in red). The speed of the CPU is a function of the Moore's law, so to speak. As a result, the number of VMs that can be supported is a function of the CPU speed. Memory is also a function of the CPU speed and it needs to be configured accordingly to keep a balanced system with the proper CPU-to-Memory ratio.