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There's no universal answer to prevent VDI overprovisioning, but IT should understand what resources exist and how they're used.
Virtual desktops must have access to adequate hardware resources to perform well. The problem with this, however, is that these resources come at a cost. Most organizations work to maximize their virtual desktop density per host to maximize the benefit of their hardware investment. Unfortunately, this can lead to resource overprovisioning, which negatively affects VDI performance. If taken to the extreme, resource overprovisioning can cause virtual desktops to fail.
Prevent resource overprovisioning
The good news is that there are many different things that IT professionals can do to prevent overprovisioned VDI resources. A good first step is to use a performance monitoring tool to get a sense of the resources that each virtual desktop consumes. Once IT pros establish a resource consumption baseline, they can determine the maximum number of virtual desktops that each virtualization host can comfortably support.
As IT determines how many virtual desktops each VDI host can handle, it is important to remember that IT can't allocate all of the host's resources. The underlying hypervisor needs memory, storage and CPU resources of its own in order to function. VDI environments are also prone to demand spikes. Over-allocating host resources may leave the host with insufficient resources to absorb demand spikes, such as the spike that inevitably occurs when users first log in for the day.
As IT pros use a performance monitoring tool to establish a resource consumption baseline, they should pay attention to which resources are in the shortest supply, compared to the demand for those resources. There will usually be one bottleneck resource that acts as the limiting factor in determining the total number of virtual desktops that can be hosted. A host might, for example, have enough CPU resources to host 100 virtual desktops, but only have enough memory to host 25 virtual desktops. In that example, the memory is the limiting factor.
In most virtual desktop environments, the resource that is in the shortest supply is storage IOPS. Therefore, it is important to get a sense of the load that each virtual desktop is placing on the storage subsystem, especially during demand spikes. IT can compare the demand for IOPS per virtual desktop against the total number of IOPS that the storage subsystem can deliver. This will help IT to figure out the maximum number of virtual desktops that the storage subsystem can handle, and therefore help to avoid the overprovisioning of storage resources.
Common overprovisioning missteps
One of the mistakes that IT pros can make is to treat this type of capacity planning as a one-time process. IT should ideally revisit the performance monitoring process every few weeks, because the demand created by virtual machines inevitably changes over time. New applications or operating system versions often cause resource requirements, but there is a variety of other potential causes. Something as simple as changing a configuration setting, installing a software patch or adopting a new security policy can change virtual desktop resource consumption. Changes to the virtual desktops aren't the only drivers of VDI resource fluctuation. As the nature of the users' jobs evolves over time, that evolution can increase or decrease virtual desktop resource consumption.
As IT protects virtual desktops against resource overprovisioning, it is also important to consider whether the hypervisor has been configured to perform dynamic hardware allocation. For example, some organizations dynamically allocate memory to virtual machines in an effort to increase virtual desktop density beyond what static memory allocation would allow.
It is possible to dynamically allocate memory and other resources safely, but it is important to avoid being overly aggressive with dynamic allocations. Otherwise IT can starve virtual desktops of resources. IT pros should start small and gradually make changes over time in an effort to see how those changes affect their virtual desktop environment.