Version 3.1 of the open-source Xen hypervisor, official word of which is imminently expected, now runs natively in 64-bit, making it possible to run any supported operating system in 64-bit or 32-bit mode, including key 64-bit applications like Oracle, SAP, Microsoft Exchange 2007 and Microsoft SQL Server 2007.
That's somewhat of a coup for open source Xen virtualization, which in other respects has lagged behind VMware's ESX hypervisor. VMware officially added 64-bit guest support to ESX this fall with the launch of version 3.0.1, even though the hypervisor itself runs as a 32-bit application.
VMware's ability to host both 64-bit and 32-bit guests comes from the separating out the 32-bit VMkernel, which handles things like scheduling and device drivers, and the 64-bit Virtual Machine Manager (VMM) that support execution of guests.
ESX's ability to run 64-bit guests is "a remarkable hack, really," said Simon Crosby, CTO at XenSource, the leader of the Xen virtualization project. "I'm amazed that it works."
Xen 3.1, however, is not the first x86 hypervisor to run native 64-bit; that distinction seems to fall to Virtual Iron Software in Lowell, Mass., which released its 64-bit platform last fall to accommodate next generation 64-bit apps. "That's the real driver – applications," said Chris Barclay, Virtual Iron director of product management.
Beyond running modern 64-bit operating systems and applications, the other main benefit of moving to 64-bit is expanded memory – a particularly relevant concept in the world of memory-constrained virtual machines. A 32-bit operating system can address only 4 GB of physical memory, although Physical Address Extension (PAE) support in Intel architecture chips can bring that number up higher.
At 32-bit, Xen was limited to 16 GB of RAM per physical host, Crosby said. In contrast, 64-bit environments can address up to 16 TB of memory. So with previous versions of Xen, running "up to ten virtual machines per host is no problem," Simon said, but depending on how much memory you allocate to a VM (virtual machine), 16 GB of RAM can be quickly consumed by more VMs.
Gordon Haff, principal analyst at research firm Illuminata in Nashua N.H. expects VMware will eventually move ESX to 64-bit, although for now, "it hasn't been a limitation." ESX currently supports a maximum of 64 GB of RAM per physical host.
"They will have to at some point," Haff said. "The day will come when 64 GB doesn't seem like a lot of memory."
Other Xen 3.1 goodies
Xen 3.1 brings another notable feature to the table: live relocation of Windows guests. Analagous to VMotion in the VMware world, live relocation allows a running virtual machine to move between physical hosts with no downtime.
Previously, Xen supported the live relocation only of paravirtualized Linux guests such as Red Hat Enterprise Linux (RHEL) 5 or Novell SUSE Linux Enterprise (SLES) 10. Now, to relocate a VM, Xen relies on the presence of hardware assisted virtual machines (HVM), i.e., Intel-VT and AMD-V chips. "The HVM technology had to land before we could offer [live relocation] for all guests," Crosby said.
Finally, Xen 3.1 includes the XenAPI, which first appeared in Xen 3.0.3. A management interface, the idea behind XenAPI is to provide third parties with "a consistent interface with which to manage Xen hosts," Crosby said. Based on XML-RPC, XenAPI will expose information about host and guest configurations, for example, or allow developers to programmatically configure storage, networking or security policies.
"It's an ecosystem-building approach," XenSource's Crosby said. "The idea is to reduce cost and the entry effort for the [independent software vendor] community."
XenSource will incorporate Xen 3.1 in its next release. Red Hat, which also uses the Xen code base, will incorporate Xen 3.1 in its next full release of RHEL, version 5.1 due out in the fall.
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