Xen is a "hypervisor" that lets a single computer run several operating systems simultaneously, using an idea called "virtualization." This enables companies to use a single server more efficiently--something that could save them money. Now "containers," a higher-level virtualization approach that makes a single operating system look like many, is also getting traction.

High ImpactWhat's new:
Novell and Red Hat have concrete plans to build "container" virtualization into their Linux products

Bottom line:
The virtualization technology, which makes a single operating system look like many, could increase the efficiency of server use and so save companies money--but it does have some limitations.

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Specifically, containers are likely to appear in the next major versions of Red Hat Enterprise Linux (RHEL) and Novell's Suse Linux Enterprise Server (SLES). The technology could even be added before those updates, company executives said.

Two projects are under way to bring containers to Linux: Vserver and OpenVZ, the latter backed by a company called SWsoft. Overall, their prospects look bright.

"I think the big advantage of a containers approach, compared to a hypervisor, is a lot less overhead. You get much higher performance," Gabriel Consulting Group analyst Dan Olds said.

Containers are increasingly popular. Sun Microsystems introduced its own container technology in 2005 with Solaris 10. And Microsoft is working on an adaptation of existing technology.

They are not suited to all tasks. Containers require all applications to use the same copy of the underlying operating system, for example. Xen and the established virtualization leader, EMC's VMware, don't have that requirement. Nevertheless, containers are desirable.

Next on the agenda
"It's something that we want to see happen," Red Hat's chief technology officer, Brian Stevens, said in an interview here during the LinuxWorld Conference & Expo. Red Hat hasn't decided whether to use OpenVZ or Vserver, he added.

Xen is the priority for RHEL 5, due to arrive at the end of the year, but after that will come containers, Stevens said. "I'm looking at that as a RHEL 6 thing," he said.

Novell, which wants to maintain Suse's reputation as the first place to find advanced new features for Linux, is more eager and is considering adding OpenVZ in Service Pack 1 of SLES 10. "We are still evaluating if this is something we can take into SP1," said Holger Dyroff, vice president of Linux product management.

If containers don't arrive with SLES 10 Service Pack 1, Novell will urge SWsoft to work with Linux programmers so that the software can be easily added to SLES 11, Dyroff said.

Debian Linux, a noncommercial version of the open-source operating system, added OpenVZ to its "Sid" development version in August.

And some work being done for Xen will help pave the way for containers. Specifically, this will provide management tools that let customers start, stop and otherwise control virtual machines. The same technology can be used to control containers, Stevens said.

"It'll be a lot easier next time. We'll be able to just plug it in. There already will be tools to manage it," Stevens said.

But SWsoft, the company that is sponsoring the OpenVZ and that sells a fuller-featured commercial version called Virtuozzo, sees things the other way around. Last week, the company announced that its container management tools will also be able to manage Xen virtual machines, said Chief Executive Serguei Beloussov.

On a diet
The main reason to use containers is because they require fewer computing resources than full-fledged virtual machines.

For example, the many components of an operating system and its applications must be loaded into memory only once, and multiple containers can use the same copy. However, containers still need unique memory for their own data storage.

Another advantage is that some processor resources are used more efficiently with containers, said Don Becker, Penguin Computing's CTO and a Linux supercomputing expert. For example, a chip's translation lookaside buffer (TLB), which converts an operating system's memory addresses into the physical locations that the computer actually uses to fetch needed information, is dramatically more useful with containers, he said.

Sun's containers, also called "zones," take only an extra 60MB of memory apiece, said John Clingan, a Sun engineer who has seen just how many he can squeeze onto a single server.

"Zones are very lightweight," he said. "I fit 600 containers running Apache on an 8GB four-processor box," referring to Web server software.