Before learning about the VMware vSphere Server, you should know what the difference between traditional physical architecture and virtual architecture is.

The preceding diagram describes the differences between a virtualized and a non-virtualized host. In traditional architectures, the operating system is directly installed on hardware devices, for example, a rack-mount server, a blade server, and so on. The operating system, which is a Microsoft Windows platform or Linux platform, can only allocate the physical CPU and memory resources. It sends and receives data on a physical network adapter. It is required to upgrade the hardware if the administrator wants to allocate more physical resources, for example, the CPU core, memory, number of Host Bus Adapters (HBAs) and network adapters, and so on. Also, it is required to schedule the service down during hardware upgrades on each ESX host.

In virtual architectures, the operation system is installed on the hardware through a thin layer of software, called the virtualization layer or hypervisor. VMware vSphere is a hypervisor that can dynamically allocate physical hardware resources to each virtual machine. For example, suppose that a vSphere Server has 64 GB memory and three virtual machines are running on this vSphere host. Each VM is allocated 4 GB, which shares the physical memory (64 GB) of that vSphere host. It is not required to upgrade the hardware if the administrator wants to allocate more memory resources to the virtual machine, because the vSphere still has 52 GB of free memory available. To sum up, virtual architecture is more flexible than traditional architecture.