With virtualization's popularity soaring, it was predictable that hardware vendors would eventually bring to market specialized servers that cater to the needs of virtual machines. The market leader in this category is Dell, which currently offers three models of virtualization-optimized systems: the entry-level R805 server, and the larger R900 and R905 servers. Although these systems make perfectly good generic servers for all standard IT uses, they have specific features that endear them to virtualization users.
First, the machines provide extensively scalable RAM. One of the most pressing constraints to consolidating systems on a virtualization platform is RAM. This is due in part to the tendency of hypervisors to allocate the VM's full configuration of RAM, even if little of it is actually used; having lots of RAM is a boon to host systems. (And likewise, configuring VMs to have the minimum amount of RAM they need is a good practice.)
Second, the servers offer substantial network I/O. An important limitation of VM hosts is the amount of disk I/O they can support. Two VMs with heavy disk I/O can essentially freeze out the other VMs if the disk storage is local. For this reason (and others), VM hosts generally don't rely on local storage but have big pipes pointing to spindle farms. To get good performance, the connection is via Gigabit Ethernet or Fibre Channel; a good host provides many adapters for network I/O.
Finally, the systems have a means of embedding the VMware ESXi virtualization software directly in the hardware, which I'll discuss later. (The embedded virtualization can be disabled and another vendor's hypervisor layer, such as products from Citrix or Microsoft, can be used instead.) This embedded option makes it easy to configure the servers for virtualization right out of the box without the usual fuss of setting up the hypervisor and the attendant utilities. In addition, in the event of hard-disk failure, the virtualization software is still available separately.
As I mentioned earlier, Dell combines these features into systems aimed at sites that use virtualization, whereas other enterprise vendors such as Hewlett-Packard put these features to varying extents into various models in their product line and deem them all "virtualization ready."
I examined two virtualization-oriented servers from Dell, the R805 and R905 systems, which use AMD Opteron processors. (In Dell marketing numerology, servers and workstations ending in "05" use AMD processors, while those ending in "00" use Intel chips.) They deliver very good performance as well as the vaunted expandability and substantial networking I/O that VM hosts so badly need.