Logical Domains: A multicore approach to virtualization

Could virtualization take advantage of the wave of multicore processors now being offered? Sun Microsystems has developed a technology, Logical Domains (LDoms), that can be used to dedicate each core of a processor to running its own operating system.

When most people think of Sun's virtualization, they probably think of Sun Solaris Containers. With this form of virtualization, a server runs a single instance of Solaris, which an administrator could add additional virtual zones, each of which can run additional operating system-based virtual environments.

LDoms takes a different approach, one that in the right circumstances could be more efficient than the Containers approach.

'Logical Domains is another form of virtualization,' said Harry Foxwell, a senior system engineer at Sun Federal, in a presentation at last year's Washington Open Solaris Users Group meeting.

Sun started offering LDoms with OpenSolaris 10 build 61 and Solaris 10 11/06, to better harness its multicore, multithreaded UltraSparc processors. The current UltraSparc T2 has eight cores, and each core is capable of handling eight threads simultaneously.

LDoms was designed to run a separate virtualized environment with each thread. As a result, A single Sun UltraSparc T2-based server can run as many as 64 virtualized instances.

Like the virtualization approaches from Advanced Micro Devices and Intel, LDoms is based on hypervisor management, or a thin layer of software that interacts with the hardware directly. However, the LDoms hypervisor does not reside on the CPU. It resides in the firmware of the machine.

Unlike Container Zones, each LDoms gets its own operating system kernel. The administrator sets up a control domain which can be used to manage the other domains, though each domain interacts directly with the hardware, eliminating the middle layer of software that slows performance in software-based virtualization approaches.

There are a number of advantages to this approach, Foxwell said. One is what he called complete CPU isolation. A heavy load in a virtual environment will not hamper the performance of the others.

Yet another benefit is a fine-grained flexibility of resources. Administrators are not limited to dedicating only a single thread to a single virtual environment.
Administrators can allocate multiple CPUs for each domain. They can also shift resources among different domains on the fly.

'You can shift back and forth the CPUs from one domain to another,' Foxwell said. 'If I make a mistake and over-provision one and under-provision another, I can use the command line or even a script to shift resources back and forth between the two.'

There are additional benefits for Sun shops, too. Foxwell said Sun's Network File System cannot run within a Container, because it requires global control of the server itself. However, an NFS file server can run within an LDoms.

LDoms supports virtualization of Solaris, OpenSolaris and Ubuntu Linux. For more information on LDoms from Sun Microsystems, see GCN GCN.com/891.

About the Author

Joab Jackson is the senior technology editor for Government Computer News.

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