Red Hat drops Xen from RHEL

With Wednesday's beta release of its flagship operating system, Red Hat Enterprise Linux (RHEL), Red Hat has added a number of new capabilities that should help data centers better support virtualization and cloud computing.

RHEL 6.0 will also have at least one less feature as well. This will be the first version of the OS not to include the Xen hypervisor. Instead the company plans to focus its virtualization efforts around the kernel-based Virtual Machine (KVM), said Tim Burke, Red Hat vice president of platform engineering.

For RHEL 6, "Virtualization has been a key focus, as has providing infrastructure that will be part of our cloud services," Burke said.

To help in cloud deployments, the RHEL 6 OS has the ability to dynamically allocate kernel data structures. "This will allow cloud service providers to give better service-level agreements," Burke said. As virtual machines are loaded on to the OS, the administrator can specify how much memory, how many processing cycles and how much network bandwidth can be allotted to each machine.

Another new addition is the Completely Fair Scheduler (CFS), which "more dynamically balances the workloads among the tasks," distributing the CPU resources more evenly across all the applications. Borrowing techniques from Red Hat's software for running latency-intolerant services, it also does a more sophisticated job of scheduling high-priority processes over low-priority ones, Burke said.

Power savings features have been added. The timing infrastructure has been reorganized as well, and uses something called the tickless kernel enhancement. Previously, the kernel would interrupt the CPU 1,000 times per second to take a time measurement, which prevented the CPU going into power-saving sleep mode. The tickless kernel feature relies instead on hardware-based timers, allowing the CPU to go to sleep in those periods when there are no other chores to complete.

The file systems space has been revamped for larger data sets. This is the first version of RHEL to use ext4 as the default file system. (Formerly it used ext3.) RHEL can now run file systems of up to 16 terabytes. The new file system also runs file system checks much more speedily, which means faster recovery times after unclean shutdowns. For really big data sets, RHEL also includes an option to upgrade to SGI's XFS file system, which can scale to 128 terabytes.

With Red Hat's emphasis on supporting cloud computing, the company's decision to drop Xen may seem surprising. But over the past few years, Red Hat has increasingly thrown its support behind KVM. In 2008, the company purchased virtualization software provider Qumranet, whose developers pioneered much of the early KVM work.

Burke said that one of the reasons Xen was dropped is that the company was duplicating a lot of effort in maintaining two hypervisors, a job that requires an increasing amount of energy. For instance, Intel added some virtualization support capabilities in its just-released Nehalem server processor, but these capabilities required some modifications in both sets of software.

Simon Crosby, a co-founder of XenSource and currently Citrix's chief technology officer for platform software, said that he wasn't surprised by Red Hat's decision to drop Xen. (In 2007 Citrix acquired XenSource and now offers commercially supported versions of the open-source Xen hypervisor.) It makes sense to only support one code base, and Red Hat has not contributed to the Xen code base for several years.

Crosby also noted that, with Xen, Red Hat did not have much luck in marketing virtualization.

"Red Hat has been lousy at enterprise virtualization. They failed at bringing Xen to the market. Now they're five years behind the rest of the community," Crosby said.

Crosby also noted that Red Hat's embrace of KVM goes in an opposite direction of others in the server-based virtualization market, notably Citrix and VMware. Both of those companies are embracing low-level virtual infrastructure, in which all the roles of the server are virtualized. In contrast, KVM relies on the OS kernel, and will only run Linux-based virtual machines. "It's a Linux-first mentality," Crosby said.

In any case, the move to KVM will necessitate some work on the part of current RHEL users to move their Xen virtual machines over to KVM ones (or require them to install Xen separately). "If you want to run RHEL 5 Xen guests on RHEL 6, you have to run a migration conversion tool on the guest," Burke said.

Burke did not disclose the final shipment date of RHEL 6, though it would be in a matter of months, he said.

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