When we declared VMware's ESX virtual machine platform to be the performance winner against Micosoft's Hyper-V - readers asked, "How could you not test Xen from either Novell or Citrix?"

The short answer then was that neither vendor was ready to enter its Xen hypervisor derivative when testing was conducted last summer. However, in the second round of identical testing done late last fall, we tested Citrix XenServer 5.0, Novell's Xen 3.2 and Virtual Iron 4.4. Two other vendors -- Sun and Red Hat -- were invited to participate but because of varying timing problems, declined to participate.

Our testing confirmed some readers' assertions that open source Xen is a formidable challenger to the closed code VMware and Microsoft hypervisors. When we measured the performance of business transactions running atop the hypervisors, Citrix's XenServer 5.0 was the top finisher in nine out of 12 test runs.

The disk I/O battle was won by Novell's SUSE Xen, which killed all competition in every contest. That achievement boils down to the fact that within the default installation we tested, Novell's SUSE Xen caches writes when using the default, file-backed disk configuration. This caching gives Novell unprecedented speed. But for some, caching disk writes bucks a longstanding practice of passing disk writes immediately to media for the purpose of maintaining transactional integrity. The counter argument there is that should a transactional failure occur while a disk write is in cache storage (before being written to disk), the problem can be easily trapped and dealt with by transaction oriented applications like databases.

When you pull in the numbers recorded by Microsoft and VMware in the last round of testing, you can see that in terms of performance, the Brothers Xen provide new and formidable competition for both hypervisor market leader VMware ESX and its more recent competitor, Microsoft's Hyper-V.

Para vs. full virtualization

Novell SUSE Xen and Citrix XenServer (along with Hyper-V) are capable of bringing into play a process called paravirtualization that can, where supported in both the host hypervisor and in the virtualized guest operating systems, enable a greater bonding between a guest VM and the resources of the physical server. With this bond in place, the guest operating system is supposed to be able access the resources of the host machine more efficiently.