Even in the best of circumstances, VDI capacity planning is not an exact science.
Underestimating VDI resource requirements can mean lackluster performance and a poor end-user experience. IT can also incur unanticipated costs trying to correct these problems.
On the other hand, overestimating virtual desktop resource requirements wastes hardware resources, reduces the virtual desktop density for VDI host servers and increases costs. As a result, the pressure to accurately estimate VDI system requirements is significant.
So, when IT undertakes VDI capacity planning for a virtual desktop deployment, it is critical to determine the hardware resources the deployment will require to run the intended user workload, including desktop operating systems and applications.
One of the most common mistakes that administrators make when it comes to VDI capacity planning is relying too heavily on capacity planning tools. While there are some excellent tools available to help, the tools are only as good as the data IT can give them. If an administrator fails to provide such a tool with accurate information, then the tool's capacity planning estimates will be skewed.
There are three main considerations administrators must account for when it comes to VDI capacity planning. These include estimating the resources the virtual desktops require, preparing for demand spikes and planning for the future.
Estimate virtual desktop resource requirements
It is probably unrealistic to expect to use a standard virtual desktop for every user in an organization. Even if everyone in an organization uses the exact same virtual desktop image, some users will inevitably consume more resources than others because of the way they work.
What IT can do, however, is create a limited number of standardized virtual desktops that it can use for capacity planning. For example, IT might create a virtual desktop configuration for the finance department, and another for the marketing department. These virtual desktops will contain the applications that users within those specific departments would work with.
From there, IT can estimate the number of people who will use each type of virtual desktop and use performance monitoring to get a feel for resource usage based on the anticipated use patterns. IT can then use this information to estimate the resources it will need to run the virtual desktops in a production deployment.
Prepare for demand spikes
Demand spikes occur when users place a heavier than normal demand on virtual desktop resources. For example, demand spikes are common in the morning when everyone signs on for the day and in the afternoon when everyone returns from lunch. The virtual desktop hosts will need to have enough reserve capacity to handle these types of demand spikes. IT should be able to get an accurate idea of the potential effect of a demand spike by monitoring a virtual desktop's resource usage at logon and during periods of heavy activity.
On a similar note, IT should make sure that the VDI deployment supports high availability for virtual desktops. Each VDI vendor has its own way of making virtual desktops highly available, but high availability is usually based on the use of failover clusters.
Failover clusters work by enabling virtual desktops to fail over to another host in the event that the host that was originally running those virtual desktops drops offline. The caveat to this is that the remaining hosts in the cluster must have sufficient capacity to run the virtual desktops that were already running on the failed host.
In other words, IT can expect to run every host at its maximum virtual desktop density and still have high availability for virtual desktops. High availability requires that adequate resources be available to transition virtual desktops from a failed host.
Plan for the future
Even if IT pros execute VDI capacity planning perfectly, virtual desktop deployments are anything but static.
In the future, virtual desktops are likely to run a newer operating system, newer application versions and perhaps even some new applications that the organization does not use today. Similarly, it's difficult to anticipate when the organization might decide to hire additional employees, each of whom will require a virtual desktop. In any case, IT should leave some resources available, or at least have an easy way of adding additional resources, for new or updated applications in the future.