VMware ESXi Hypervisor

The core of the vSphere product suite is the hypervisor, which is the virtualization layer that serves as the foundation for the rest of the product line. In vSphere 5 and later, including vSphere 5.5, the hypervisor comes in the form of VMware ESXi.

Long-time users of VMware vSphere may recognize this as a shift in the way VMware provides the hypervisor. Prior to vSphere 5, the hypervisor was available in two forms: VMware ESX and VMware ESXi. Although both products shared the same core virtualization engine, supported the same set of virtualization features, leveraged the same licenses, and were considered bare-metal installation hypervisors (also referred to as Type 1 hypervisors; see the sidebar titled “Type 1 and Type 2 Hypervisors”), there were still notable architectural differences. In VMware ESX, VMware used a Linux- derived Service Console to provide an interactive environment through which users could interact with the hypervisor. The Linux-based Service Console also included services found in traditional operating systems, such as a firewall, Simple Network Management Protocol (SNMP) agents, and a web server.

Type 1 and Type 2 Hypervisors:

Hypervisors are generally grouped into two classes:

1. Type 1 hypervisors

2. Type 2 hypervisors.

Type 1 hypervisors run directly on the system hardware and thus are often referred to as bare-metal hypervisors.

Type 2 hypervisors require a host operating system, and the host operating system provides I/O device support and memory management. VMware ESXi is a Type 1 bare-metal hypervisor. (In earlier versions of vSphere, VMware ESX was also considered a Type 1 bare-metal hypervisor.)

Other Type 1 bare-metal hypervisors include KVM (part of the open-source Linux kernel), Microsoft Hyper-V, and products based on the open-source Xen hypervisor like Citrix XenServer and Oracle VM.

VMware ESXi, on the other hand, is the next generation of the VMware virtualization foundation. Unlike VMware ESX, ESXi installs and runs without the Linux-based Service Console. This gives ESXi an ultralight footprint of approximately 70 MB. Despite the lack of the Service Console, ESXi provides all the same virtualization features that VMware ESX supported in earlier versions.

The key reason that VMware ESXi is able to support the same extensive set of virtualization functionality as VMware ESX without the Service Console is that the core of the virtualization functionality wasn’t (and still isn’t) found in the Service Console. It’s the VMkernel that is the foundation of the virtualization process. It’s the VMkernel that manages the virtual machines’ (VMs’) access to the underlying physical hardware by providing CPU scheduling, memory management, and virtual switch data processing.