The benefits of virtualization are many and often loudly trumpeted. Though they are hardly mentioned or discussed, it would be a mistake to think that virtualization doesn't have some downsides as well. So what is it that you will not find the salesman telling you when it comes to virtualization? I attempt to list some of them below:
The self-contained nature of virtual machine images is an advantage that is often brought up when talking about virtualization. Indeed, a single hard disk could well hold the virtual machine images required for the operation of the entire small and medium business. This is certainly advantageous from a disaster recovery and business continuity point of view.
On the other hand, have you considered the security conundrum inherent to such readily accessible images? Remember, most virtual machines run directly off centralized SANs for greater performance. Staffers with the appropriate rights to the SAN could theoretically make unauthorized copies of entire virtual machines of key systems or systems containing confidential data without anyone being the wiser.
On virtual server sprawl
I made a mention of virtual server sprawl in my earlier piece where I discussed some signs that your SMB is not ready for virtualization. This can be a more common problem than you think, especially for SMBs that have acquired per-seat or site licenses for their operating systems or other software.
Obviously, it would be foolish to forgo virtualization over concerns of virtual server sprawl. On the other hand, it would also be illogical not to acknowledge the possibility of this problem, and take steps to preempt it. One proactive step that can be taken entails the drawing up of clear policies pertaining to the creation and maintenance of virtual machines.
On the ability of data centers to cope
Current data centers are already burdened by the exacting power requirements of densely packed 1U or 2U rack servers, as well as blade servers. Multi-core processors certainly added to the equation, without even considering that virtualization is driving processor utilization to an all-time high.
In addition, server hardware used to host virtual machines tends to generate a lot more heat. Of course, spacing the servers out might relieve heat issues, though I'd just as soon leave out the savings in rack space from my ROI projections, if I were you.
On performance measurement
It used to be easy to determine the amount of performance required to run a typical application. If the processor utilization shows half full, it means that the server can probably handle twice as many clients; full utilization most of the time means it's time to get another server.
In recent years, the advent of multi-tier Web applications has only made things more complex. In addition, virtualization added a number of other factors to consider, such as hypervisor overhead, and the amount of disk and network I/O that is available, among others. Add in the latest generation of live migration technologies such as VMware's VMotion that can dynamically load balance virtual machines between multiple physical servers, and you have a huge problem when it comes to measuring the performance of your systems.
The result is that you will be unable to pinpoint any performance bottleneck that might develop. This can get pretty ugly because only some bottlenecks can be resolved by acquiring more physical hardware.
In the meantime, are there other kinks that you have encountered in your SMB's implementation of virtualization? Feel free to share with us here.