Information Technology Reference
In-Depth Information
Long-time users of VMware vSphere may recognize this as a shift in the way VMware pro-
vides 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 con-
sidered 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 differ-
ences. 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 i rewall, Simple
Network Management Protocol (SNMP) agents, and a web server.
Type and Type Hypervisors
Hypervisors are generally grouped into two classes: Type 1 hypervisors and Type 2 hypervi-
sors. 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 oper-
ating 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 foun-
dation. 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 ear-
lier versions. Of course, ESXi 5.5 has been enhanced from earlier versions to support even more
functionality, as you'll see in this chapter and in future chapters.
The key reason that VMware ESXi is able to support the same extensive set of virtualiza-
tion functionality as VMware ESX without the Service Console is that the core of the virtu-
alization 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. Figure 1.1 shows the structure of
VMware ESXi.
We mentioned earlier that VMware ESXi 5.5 is enhanced over earlier releases. One such area
of enhancement is in the coni guration limits of what the hypervisor is capable of supporting.
Table 1.2 shows the coni guration maximums for the last few versions of VMware ESX/ESXi.
