Sizing Your Windows Server 2008 R2 Server to Support Virtualization

Although the minimum requirements for server compatibility for Windows Server 2008 R2 applies, because server virtualization is the focus of this server system, the minimum Windows Server 2008 R2 server requirements will not be sufficient to run Hyper-V virtualization.

Additionally, although Windows Server 2008 R2 supports up to 64 processor cores, 1TB of RAM, and 384 concurrently running virtual machines, the reality on the scaling of Windows virtualization comes down to the raw capabilities of network I/O that can be driven from a single host server. In many environments where a virtualized guest system has a relatively low system utilization and network traffic demand, a single host system could easily support a dozen, two dozen, or more guest sessions. Other environments where a virtualized guest session has an extremely high system utilization, lots of disk I/O, and significant server network I/O, the organization might find that a single host server would maximize its capacity with as few as seven or eight guest sessions.


RAM for the Host Server
The rule of thumb for memory of a Windows Server 2008 R2 server running Hyper-V is to have 2GB of RAM for the host server, plus enough memory for each guest session. Therefore, if a guest session needs to have 2GB of RAM, and there are three such guest sessions running on the host system, the host system should be configured with at least 8GB of RAM. If a guest session requires 8GB of memory and three of those systems are running on the system, the server should be configured with 24GB of memory to support the three guest sessions, plus at least 2GB of memory for the host system itself.


Processors for the Host Server
The host server itself in Windows Server 2008 R2 virtualization has very little processor I/O requirements. In the virtualized environment, the processor demands of each guest session dictate how much processing capacity is needed for the server. If a guest session requires two cores to support the processing requirements of the application, and seven guest sessions are running on the system, the server should have at least 15 cores available in the system. With quad-core processors, the system would need four physical processors. With dual-core processors, the system would need at least eight physical processors. With Windows Server 2008 R2 virtualization, each guest session can have up to 64 cores dedicated to the session, or processing capacity can be distributed, either equally or as necessary to meet the performance demands of the organization. By sharing cores among several virtual machines that have low processing needs, an organization can more fully utilize their investment in hardware systems.


Disk Storage for the Host Server
A host server typically has the base Windows Server 2008 R2 operating system running on the host system itself with additional guest sessions either sharing the same disk as the host session or the guest sessions virtual disks being stored on a storage area network (SAN) or some form of external storage.

Each guest session takes up at least 7GB of disk space. For guest sessions running databases or other storage-intensive configurations, the guest image can exceed 10GB, 20GB, or more. When planning disk storage for the virtual server system, plan to have enough disk space to support the host operating system files (typically about 7GB of actual files plus space for the Pagefile) and then disk space available to support the guest sessions.

Source of Information : Sams - Windows Server 2008 R2 Unleashed