Virtual servers may make the utilization of physical servers more efficient, but that efficiency comes with a price. The physical servers that support virtual machines (VMs) have to be more powerful. Less obviously, they also need better input/output (I/O) capability than standalone machines, which means more cabling to connect the virtual machines to the network switch and more switch ports to accommodate the increased number of virtual machines contending for the network's resources.

"Virtualization is about consolidation and concentration," says Bernd Herzog, an analyst at research firm Application Performance Management Experts. "It takes many disparate and distributed systems and concentrates them together onto a relatively few physical servers, and therefore onto a much smaller set of network pipes and storage area network (SAN) access infrastructure. "

According to Gartner, four million virtual machines will exist by 2009. IDC (a sister company to CXO Media) predicts that by 2010, 14.6 percent of servers shipped will host VMs.

A number of companies are addressing the impact of server virtualization on the IP and storage networks. Gear6 and Xsigo have both introduced hardware products in recent months that virtualize I/O operations, much as VMware, Citrix XenServer and Microsoft Virtual Server allow the virtualization of the server's CPU.

Each has taken a different approach to virtualizing I/O between servers and the network.

Gear6's CACHEfx appliance, which was introduced in May 2007, uses random access memory to provide a virtualized I/O environment for data that applications need to retrieve and process quickly.

As server speeds increase and as more virtual machines are placed on physical servers, applications become I/O bound.

"Pre-virtualization, IT shops had one physical server with one I/O channel," says Gary Orenstein, VP of marketing for Gear6. "There was complete synchronization between the application, the operating system, the motherboard, the I/O driver and the I/O link. As soon as they put multiple virtual machines on that link, none of them is coordinated with the other. If they had virtual machines, all the VMs were grasping for that link simultaneously without coordination."

Xsigo takes a different approach: Its I/O Director, which debuted at VMworld last fall in San Francisco, lets IT control I/O between servers and the network by creating virtual I/O channels that can be used by applications and other processes.

Whitepapers

• Symantec State of the Data Center Report
• Gene Kim's Practical Steps to Mitigate Virtualization Security Risks
• Monitoring and Managing App Performance
• Monitoring and Managing VoIP
• SNMP Monitoring: One Critical Component to Network Management
• SNMP Monitoring: One Critical Component to Network Management

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• Executive Guide: Storage Heats Up
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