At the core of any virtualization platform is the hypervisor. The VMware hypervisor is named vSphere ESXi, simply referred to as ESXi. ESXi is a Type 1 or bare-metal hypervisor. This means that it runs directly on the host's hardware to present virtual hardware to the virtual machines. In turn, the hypervisor schedules access to the physical hardware of the hosts.
ESXi allows multiple virtual machines with a variety of operating systems to run simultaneously, sharing the resources of the underlying physical hardware. Access to physical resources, such as memory, CPU, storage, and network, used by the virtual machines is managed by the scheduler, or Virtual Machine Monitor (VMM), provided by ESXi. The resources presented to the virtual machines can be over committed; this means more resources that are physically available can be allocated to the virtual machines on the physical hardware. Advanced memory sharing and reclamation techniques, such as Transparent Page Sharing (TPS) and ballooning, along with CPU scheduling, allow for over commitment of these resources to be possible, resulting in greater virtual-to-physical consolidation ratios.
ESXi 6.7 is a 64-bit hypervisor that must be run on a 64-bit hardware. An ESXi 6.7 installation requires at least 1 GB of disk space for installation. It can be installed on a hard disk locally, a USB device, a Logical Unit Number (LUN) on a Storage Area Network (SAN), or deployed stateless on hosts with no storage using Auto Deploy. The small footprint of an ESXi installation provides a reduction in the management overhead associated with patching and security hardening.
With the release of vSphere 5.0, VMware retired the ESX hypervisor. ESX had a separate, Linux-based service console for the management interface of the hypervisor. Management functions were provided by agents running in the service console. The service console has since been removed from ESXi, and agents now run directly on ESXi's VMkernel.
To manage a standalone host running ESXi, a Direct Console User Interface (DCUI) is provided for basic configuration and troubleshooting. A shell is available that can either be accessed locally from the console or remotely using Secure Shell (SSH). The esxcli command-line tools and others can be used in the shell to provide advanced configuration options. An ESXi host can also be accessed directly using the vSphere Client. The ESXi DCUI is shown in the following screenshot:
