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Virtual desktops and virtual servers use hypervisors, but there are many differences between server and desktop virtualization, including how memory and workload type affect density.
First, it's often possible to achieve a higher density for virtual desktops than virtual servers. The operating systems and applications you use make a major difference in the number of virtual servers or virtual desktops a physical server can host. Let's begin with a comparison of minimum system requirements.
You'll notice Windows Server 2012 R2 and Windows 8.1 (64-bit) have somewhat similar system requirements. Windows Server requires a slightly faster CPU and a little more storage, but surprisingly Windows 8.1 needs four times as much memory as Windows Server:
|Windows Server 2012 R2||Windows 8.1 (64-bit)|
|CPU||1.4 GHz, 64-bit||1 GHz or faster with support for PAE, NX, and SSE2|
|Memory||512 MB||2 GB|
|Storage||32 GB||20 GB|
So how is it possible to achieve a much higher density of virtual desktops than virtual servers?
A host server can generally accommodate more virtual desktops than virtual servers because of how the host uses the virtual machines (VMs). The specs listed in the chart above represent the minimum supported hardware. The minimum hardware for Windows 8.1 would likely allow for a decent user experience. Conversely, the minimum hardware requirements listed for Windows Server 2012 R2 are barely enough to boot the server. To run any sort of workload on the server, you'll need additional hardware.
Another way of looking at these differences is in the types of work the VMs perform. A virtual server typically hosts resources -- such as a file share or an application -- that multiple users access simultaneously. Therefore the virtual server must be equipped with enough resources to accommodate all the anticipated user sessions. A Windows 8.1 desktop only has to accommodate one user at a time, however.
Why virtual desktops get better density
Obviously server and desktop virtualization are different, but you may still be wondering why more virtual desktops than virtual servers can run on a host server.
First, there's no rule that says a host server can't accommodate many virtual servers. In theory, Hyper-V and VMware ESXi are capable of running roughly 1,000 virtual servers on a single host. As such, it is generally hardware that limits the number of VMs you can host, not software.
Memory is usually the limiting factor for virtual desktops. A server with 128 GB of RAM could host 64 Windows 8.1 desktops where each virtual desktop consumes 2 GB of RAM. The host OS needs resources too, so the actual number of virtual desktops you can host is probably closer to 60.
Memory can put a ceiling on virtual servers too. It's not uncommon for a single virtual server to require 32 GB or more of RAM (because of application requirements). But memory is not the only restraint. Virtual servers are usually assigned multiple virtual CPUs, whereas it's common for a virtual desktop to use only a single virtual CPU.
Ultimately the workload determines the number of virtual machines a server can host. A virtual desktop doesn't consume a lot of resources if the user only works with a word processor or a Web browser. Such a virtual desktop may be idle most of the time. As a result it's often possible to cram dozens of virtual desktops onto a single host. If users work with something more demanding, such as a 3D CAD application, then the number of virtual desktops a host could accommodate would sharply decrease. Workload size puts the same limits on virtual servers.
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