Hypervisors and Virtualization

If you’re deep into the world of virtualization, you probably know what a hypervisor is, what it does, and why one type of hypervisor is better than another.  But to the uninitiated, hypervisor is just another term to describe an element in the complex world of virtualization.  To find the right virtualization technology for a specific purpose, it is helpful to understand a bit more about hypervisors and why these “electronic supervisors” matter in the world of virtualization.

CIO.com states that “…virtualization refers to technologies designed to provide a layer of abstraction between computer hardware systems and the software running on them. By providing a logical view of computing resources, rather than a physical view, virtualization solutions make it possible to do a couple of very useful things: They can make your operating systems think that a group of servers is a single pool of computing resources.  And they can allow you to run multiple operating systems simultaneously on a single machine…”

A hypervisor is a term used to describe software that acts as a virtual machine manager.  It is not a new concept.  IBM developed it back in 1967.  The type 1, or bare-metal hypervisor, runs directly on physical hardware without the overhead of an OS.  Type 1 hypervisors are considered the gold standard for enterprise-class hypervisors.  Type 2 hypervisors run within a conventional operating system (OS), that is, there is an OS between the hypervisor and the physical hardware.

VMware’s ESX software continues to be the industry leading hypervisor in terms of market share.  VMware has been in the arena for over 10 years with tens of thousands of clients, including 100% of the Fortune 500 companies.  Since its release in 2001, ESX has grown to become the most used hypervisor in the market.

Microsoft released its own hypervisor, Hyper-V, last June.  Between that and their legacy Virtual Server 2005, Microsoft’s virtualization market share (by license count) increased to nearly 25% last year.  Microsoft recently released its own type 1 hypervisor (Hyper-V) that sits on a small Windows kernel rather than Linux.

Citrix’s XenServer uses the Xen hypervisor.  Also used in Sun’s (now Oracle’s) xVM product among others, Xen is an open-source virtualization engine.  Rather than develop their own hypervisor, Citrix acquired the maker of XenServer and XenSource in late 2007.  XenServer is free, but Citrix sells a premium management tool called Citrix Essentials that extends the capabilities somewhat.  XenServer is a type 2 hypervisor but Citrix plans to release a type 1 version of the hypervisor sometime in 2009.

So what’s the difference between these competing technologies?  VMware is the industry giant and VMware’s ESX is a ‘bare metal’ installation.  At only 32 MB in size, ESX is optimized to manage the Virtual Machine I/O and hardware translation with very little overhead.  ESX can over-allocate resources such as memory, enabling direct control over the physical resources.  XenServer still requires a base OS and Hyper-V only recently released its bare-metal hypervisor.  With type 2 hypervisors, the installed operating system manages the hardware, similar to how a server functions.  As a result, type 2 hypervisors like XenServer have more drivers available to them, but require a layer of complexity that is absent in a type 1 offering like VMware’s ESX.  As with many disruptive technologies, the virtualization landscape continues to shift and evolve at a rapid pace, so don’t blink or you might miss an important development.

Tags: ESX, Hyper-V, hypervisor, virtualization, VMWare, XenServer, XenSource