Red Hat's KVM Surpasses Xen, Aims at VMWare
Xen Destined for the Scrap Heap
Red Hat got its hands on KVM - which works in conjunction with Intel and AMD chips with hardware virtualization support - when it acquired Qumranet, an Israel-based company, in September 2008. While the company continues to support Xen for its existing Red Hat Enterprise Linux (RHEL) 5 customers, it is being positioned very much as a legacy virtualization solution. "We believe KVM represents the next generation of virtualization," said Thadani. "KVM offers significant benefits over Xen and proprietary systems and is the strategic direction we want to go, so we will encourage our customers to start to migrate to KVM when they are ready to, and provide the tools and services needed to help them do so."
So what's so great about KVM that has caused Red Hat to prepare to abandon Xen? Most of the attraction stems from the fact that KVM is part of the Linux kernel, rather than being a separate kernel, as is the case with Xen. That means all the benefits of the thousands of hours of work done on the Linux kernel - including scalability, robustness, security, memory management, NUMA, and the vast hardware ecosystem supported - also accrue to the KVM hypervisor automatically, rather than needing to be done again. As a result, KVM can immediately support up to 96 cores and a terabyte of RAM at the host level. (Individual guest OSes can have up to 16 virtual CPUs.)
As an example of a benefit of using KVM, Thadani mentioned memory page sharing. "Memory is usually the binding constraint when it comes to virtualization. We already have memory page sharing in KVM, while this is much harder in Xen, so we can run more virtual machines on any given server with KVM," he says.
Thadani suggests another example: using KVM, every running virtual machine is a separate Linux process on the host machine. This means "bare metal" Linux apps and virtual machines can easily run side by side on the same physical host. This could be very useful in a data center that, during business hours, must run business applications that can't be virtualized, he explained. After hours, these same physical machines could be put to work playing host to a larger number of virtual machines.
Red Hat plans to provide KVM in two ways: By offering KVM with Red Hat Enterprise Linux (RHEL) 5.4, which was released in early September, or as a separate thin Red Hat Enterprise Virtualization (RHEV) hypervisor. This is effectively a KVM hypervisor that runs on bare metal, like VMware ESX Server or Xen, rather than a full RHEL installation that offers virtualization.
Which is the most appropriate offering?