It seems the definition of just enough operating system (JeOS) and the amount of OS needed for virtual appliances depends on who you talk to. The term, coined by virtualization leader VMware, refers to an OS stripped down to only the necessary components to run a particular application or stack of software.

Ubuntu Linux distributor Canonical describes JeOS as a platform on which people are comfortable developing that also has backing of expertise and key relationships. The company says there is huge interest from ISVs in how to build virtual appliances, and it is confident its Ubuntu JeOS, announced last September, will continue to draw application developers.

Following its recent Linux Automation announcements, Red Hat is also gearing up a new version of RHEL for appliances. In doing so, it has indicated it thinks the kernel and runtime components of the OS are still very much needed. Red Hat also claims its just enough is up to the task, but is actually slimmer than Canonical’s Ubuntu JeOS. Canonical’s response: it’s hard to comment on Red Hat since it is not yet actually in the light-Linux, appliance OS market.

Another interesting one to watch is rPath. While it clearly sells virtual appliance software to ISVs, rPath sees the model evolving to an “application fabric” (451 Group term) that efficiently supports applications. This is more like a grid or a cloud such as Amazon’s EC2, where, coincidentally, rPath sells its appliances.

We’re also hearng more of Microsoft’s approach to how much OS is needed and how much is not. With Windows Server 2008, the software giant is delivering a more role-based, componentized OS. Microsoft figures the OS should be able to bend, fatten up or thin down depending on the many different potential uses it serves. Microsoft may be taking a lesson or two from open source with its flexibility and componentization, and there’s good reason for it. Linux and open source allow the creation of whatever iteration of the OS or application that best suits the appliance.

So, how much OS is enough OS? Well, the correct answer will vary, but I think Linux and open source software have advantages when it comes to providing the flexibility and customization to best suit different needs. Some like the JeOS sweet, others like it more watered down. Open source gives the users, in this case ISVs, the ability to mix it just the way their applications and users like it.