Are You Virtually Compliant?
In order to help prevent technical anarchy on a grand scale, I find myself frequently pointing out to those undertaking virtualization initiatives that it is not merely a sizing exercise. First and foremost it is an exercise in constructing a coherent infrastructure that puts the right things together and keeps the right things apart, especially when dealing with production systems. You don't build a sophisticated IT infrastructure over many years to simply have it "randomized" into VMs based on utilization.
Some counter by asking "Why not? If the systems are in their own VMs then what difference does it make?" Rather than go into the details of how problematic it can be to combine systems with different availability levels, different DR strategies, etc., I find it easiest to think of things from a security perspective.
System-level security has traditionally been based on the premise that you cannot see the local storage without first authenticating with the system at the OS level (i.e. logging on). Network-based storage authenticates at the system level and thus also prevents one from accessing it without being "on the system". This access control is like a castle wall that protects everything inside it, and is the first line of defense in any IT security strategy.
Virtualization is a different paradigm, and as these things go, new paradigms often upset existing assumptions. Most virtualization technologies store their VM images in files, and this encapsulation enables many of the benefits of virtualization by providing portability, state management, interoperability and a whole list of other advantages. But it also means that those with access to these files can bypass all authentication mechanisms on that system; by having a complete file (and memory) image of a virtual machine, you can peer inside it with little effort. The castle wall is suddenly made of very thin paper.
In fact, most VM vendors provide utilities to "mount" a VM image as a disk, providing convenient access to its contents. This means that the old fear of someone "walking off with a disk" has now become a fear of someone "copying a file", which is considerably easier to do. But more to the point of this topic, it means that any administrator or user with access to these images becomes a "super super" user, with access to the contents of systems that they may not otherwise be privy to. That fact that a single user exists that can access (and modify) the data of any app on a virtual cluster is enough to kill pretty much any compliance audit.
There are, of course, ways to prevent this. Some vendors support encryption of the VM images, although this will undoubtedly cause performance to tank. Some provide affinity and anti-affinity rules to influence the movement of images within a pool of physical systems, but offer no practical means to establish and maintain these rules. (Not to mention the fact that they are already in the same pool may make it too late to achieve true isolation.)
The best solution, in my opinion, is to simply avoid combining systems that should never go together. Seems simple enough - this is what you spent the last 10 years doing, so why stop now. And the best way to accomplish this is to establish in advance the business constraints that govern a virtualization project and ensure that these constraints are honoured throughout the process. And then look at sizing...
