VMware has put a lot of effort into the vSphere 4.1 release, and with it, VMware made it official that vSphere 4.1 is the last release from the virtualization giant that will contain both the "classic" version of ESX that we've all become familiar with, and the newer, slimmed-down version, called ESXi. From now on, all new releases of vSphere will only include the ESXi hypervisor.
VMware's classic ESX hypervisor includes a user space environment, known as the Console Operating System (COS) or Service Console, which is derived from a distribution of Linux. It's used as both a bootstrap for the VMware kernel and as a management interface that could be automated and queried against using Linux-style commands and scripts.
The COS is being deprecated as VMware moves to exclusively using the embedded or trimmed-down version of ESX, the ESXi model. This makes the hypervisor footprint substantially smaller and potentially more secure and efficient. The smaller hypervisor footprint also provides users with delivery mechanisms beyond installation on local storage such as SD cards, USB memory sticks, or even being embedded on the server hardware itself before ever leaving the factory.
But if ESXi is so great, why has VMware had its work cut out for it in convincing skeptical customers of the value from moving from ESX to ESXi? The company has been promoting it for years now, and it has even made the ESXi offering a free hypervisor. But that's been part of the problem as well. By making it free and by creating arbitrary limitations around it, the ESXi platform for many has become thought of as a low-end, low-grade version of the superior classic ESX hypervisor.
One of the most common fears among die-hard ESX community members about moving to ESXi has been the loss of the COS and the ability to script or interactively SSH into the machine in order to perform management and command-line functions.