What Is the VMware ESXi Server and Its Role in the VMware Suite?

VMware ESXi is the bare-metal hypervisor in the VMware vSphere virtualization platform. As a bare-metal hypervisor for creating and running virtual machines (VMs), VMware ESXi runs on top and accesses the hardware directly without the need to install an operating system. This direct access to hardware allows it to perform better, run faster and be more scalable than other types of hypervisors. This makes VMware ESXi ideal for use in a large-scale virtual desktop infrastructure (VDI), in conjunction with the other components in the VMware vSphere platform.

What VMware ESXi Server Means

VMWare ESXi is the name of VMware’s bare-metal or Type-1 hypervisor. A hypervisor is a special kind of operating system where you can create, run, and manage multiple virtual machines. Each VM can have its own guest operating system and one or more applications. The guest OS can be any general-purpose OS like Windows, macOS, Linux, and so on.

Thus, a single VMWare ESXi Server can support multiple Windows VMs or a combination of multiple Windows, macOS, and Linux VMs, simultaneously. This is called server consolidation, a practice that allows you to save on hardware, cooling, and power costs, as well as maximize utilization of physical resources.

Being a Type-1 hypervisor, VMWare ESXi Server runs directly on top of physical infrastructure. Hence the term ‘bare metal’. This characteristic is different from that of a Type-2 hypervisor, which has to be installed on top of an operating system. The following diagram illustrates the difference between the two.

type 1 vs type 2 hypervisor

Since VMware ESXi doesn’t run on an operating system, it has direct access to underlying resources like the CPU, RAM, storage, and networking. For this reason, VMware ESXi performs much better than any Type 2 hypervisor. Consequently, the virtual machines and the applications on those VMs run much faster than they could on a Type 2 hypervisor.

VMWare ESXi is one of the components comprising VMware VSphere, a suite of software solutions for virtualization. Other components that usually work alongside ESXi include VCenter Server, vCenter Server database, VMware Directory Service, and others.

VMware ESXi Features

VMware ESXi relies on the VMkernel, which is a microkernel, to run the hardware, guest systems and service layer making up the VMware virtualization platform. The VMkernel runs the physical hardware and guest VMs, connects directly to the CPU and memory and uses modules linked via another module to access the hardware’s networking and storage components.

Notable features of VMware ESXi include:

Small size

At just 150 MB, Vmware touts ESXi as the world’s smallest hypervisor. A small footprint often means a reduced attack surface from outside threats, aside from easier maintainability. This also means that ESXi does not need frequent patching up.

Convenient installation

With its small size, faster installation is possible, allowing you to set up your infrastructure at the soonest time possible. You can even boot up ESXi from a USB flash drive.

User-friendly administration tools

ESXi offers a built-in, HTML5-compliant browser for administrative use. Organizations that require automated operations can also utilize the vSphere Command Line Interface for remote management and application programming interfaces (APIs) based on Representational state transfer (REST).

Secure design

The data that exists in your VMs are secure from prying eyes using built-in encryption. Add role-based access plus extensive logging and auditing capabilities and you get a secure virtual platform at the outset.

Scalable reliability

No matter your application requirements, a single ESXi hypervisor can be configured to run as many as 128 virtual CPUs and 120 devices on 6 TB of memory. ESXi also features a high-performance cluster file system dubbed the Virtual Machine File System (VMFS) that allows more storage resources to be distributed, even with limited physical storage. VMs can also use more than one CPU simultaneously.

Extensive support and compatibility

ESXi’s popularity as an enterprise platform means wide support from hardware vendors and application partners, as well as compatibility with a broad range of applications and guest operating systems.

VMware ESXi Requirements

Due to its small footprint, ESXi can run on a host machine with as little requirements as:

However, for better performance, it is recommended that the host have four or more CPU cores and at least 8 GB of RAM, as well as dual gigabit or faster Ethernet adapters and redundant drives.

If your organization is running 64-bit VMs, you must enable hardware virtualization for the x64 CPUs in the ESXi server. In addition, the no-execute/execute-disable (NX/XD) bits in these CPUs must be enabled in the BIOS. NX stands for Never eXecute and is available in AMD CPUs; eXecute Disable is Intel’s version of the bit. The NX/XD bit reserves memory in the hardware’s data storage areas to prevent them from being hijacked during malicious attacks. Thus, it provides an extra layer of security to an ESXi server.

Other requirements include a Small Computer System Interface (SCSI) disk or a local and non-network, logical-unit-number configured (LUN-configured) Redundant Array of Independent Disks (RAID) with unpartitioned space for hosting the VMs. Serial ATA (SATA) disks are also supported.

ESXi hosts support booting from the Unified Extensible Firmware Interface (UEFI) on hard drives, CD-ROM drives or USB media. Network booting and provisioning of ESXi hosts with UEFI are also supported. For compatible hardware, ESXi can also boot from disks that are 2 TB or more in size.

To start using ESXi for your virtualization requirements, install it directly on the host machine’s hard disk. Installation is straightforward as well—start the installer and go through the wizard. Post-installation, you can change the host options from the console and launch VMs using the HTML5-based vSphere Client.

VMware ESXi Licensing Model and Cost

VMware ESXi is provided as part of the free vSphere Hypervisor edition. Thus, the tool itself is free to use. However, for easier manageability, you might want to use the vSphere Server, vSphere management tools and other VMware features with ESXi. This means paying licensing fees on a per-CPU basis, with a license valid for use in up to 32 physical cores. For CPUs with more than 32 physical cores, you need more licenses.

There are also a wide range of vSphere Editions with varying configurations. At the time of writing, the costs of these editions vary as well, with the least expensive costing $510 for three servers and two processors per server, and the most expensive costing $6,545. You need to pay separate fees for Basic Support and Production Support.

ESXi Server Versus Hyper-V

Microsoft Hyper-V is another type-1 hypervisor in the mold of an ESXi Server. Hyper-V was first released as a component of Windows Server 2008. It is a free and standard component in later Windows Server versions as well as 64-bit versions of the Windows 10 Pro, Enterprise and Education editions.

ESXi and Hyper-V have their own set of pros and cons. Some features common to the two systems are:


The VMware ESXi architecture mainly consists of VMkernel and the processes that run on top of it. Let’s talk about the VMkernel and some of those processes now.


VMkernel is technically an operating system. Like other operating systems, VMkernel creates and controls processes, controls hardware devices on the server, uses a file system, manages application resources, and so on. Its main function, however, is to support virtual machines.
The main processes that run on VMkernel include the following.

Direct Console User Interface (DCUI)

DCUI is the local configuration and management interface. You can only access the DCUI locally, i.e., on the VMware ESXi server itself. Its user interface is menu-driven, with a look and feel similar to that of a BIOS interface. You mainly use the DCUI for initial configuration and troubleshooting purposes.

Virtual Machine Monitor (VMM)

VMM provides the execution environment for a virtual machine. This process is always accompanied by a corresponding Virtual Machine Executable (VMX) process. This means that each running VM is always accompanied by a VMM process and a VMX process. The former is in charge of virtualizing the guest OS instructions and managing memory, while the latter manages I/O to certain devices.

Common Information Model (CIM) System

The CIM system provides an interface for remote applications so that they can access device drivers and underlying hardware resources on the ESXi server in question. Access is provided through a set of APIs. The CIM system consists of an object manager known as a CIM broker and a collection of CIM providers. CIM providers are written by VMware itself as well by hardware vendors who want a way to manage and monitor their manufactured devices.

The diagram below depicts a simplified representation of the VMware ESXi architecture. It includes VMkernel, the major processes mentioned above, and other processes like vpxa, SNMP, hostd, and syslog.

VMware ESXi architecture

ESX and ESXi Versions

ESXi isn’t the first VMware hypervisor. Before ESXi came into existence, there was VMware ESX. ESXi was first released in 2011, on the same day that ESX 4.0 Update 3 was released. The official release name was also ESXi 4.0 Update 3. ESXi and ESX coexisted until 2014, when ESX was discontinued after ESX 4.1 Patch 11a.

ESXi has the same functionality as ESX but also has a much smaller footprint. This means ESXi has a smaller attack surface and hence is more secure than ESX. The small footprint also means this hypervisor consumes less resources than ESX. Given the same hardware computing resource specifications (e.g., similar CPU, RAM, storage), an ESXi system will have more resources available for virtual machines than an ESX system.

All in all, the ESXi architecture is superior to the ESX architecture, which explains why ESX was discontinued and ESXi is now at ESXi 7.0 Update 3g as of this writing.

Parallels RAS and VMware-Based Hypervisors

Parallels® Remote Application Server (RAS) supports both VMware ESXi Server and VMware vCenter as a VDI host. With Parallels RAS having any of these two as a VDI host, you can use them to manage VMs, create guest clones and publish desktops and applications. After setting up and configuring the host, you can set up an agent on the guest VMs, together with guest pools and templates, all from a single pane of glass.

Parallels RAS also supports various other hypervisors and hyperconverged systems, including Hyper-V, Microsoft Hyper-V, Scale Computing HC3 and Nutanix Acropolis.

With Parallels RAS, you can quickly create a VDI with centralized management capabilities and better security overall.

Start setting up your VDI environment by downloading the Parallels RAS trial.

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