VMware ESXi (formerly ESX) is an enterprise-class, type-1 hypervisor developed by VMware for deploying and serving virtual computers. As a type-1 hypervisor, ESXi is not a software application that is installed on an operating system (OS); instead, it includes and integrates vital OS components, such as a kernel.
|Initial release||March 23, 2001|
6.7.0 U3 / August 20, 2019
|Platform||IA-32 (x86-32) (discontinued in 4.0 onwards), x86-64|
|Type||Native hypervisor (type 1)|
After version 4.1 (released in 2010), VMware renamed ESX to ESXi. ESXi replaces Service Console (a rudimentary operating system) with a more closely integrated OS. ESX/ESXi is the primary component in the VMware Infrastructure software suite.
The name ESX originated as an abbreviation of Elastic Sky X. In September 2004, the replacement for ESX was internally called VMvisor, but later changed to ESXi (as the "i" in ESXi stood for "integrated").
ESX runs on bare metal (without running an operating system) unlike other VMware products. It includes its own kernel: A Linux kernel is started first, and is then used to load a variety of specialized virtualization components, including ESX, which is otherwise known as the vmkernel component. The Linux kernel is the primary virtual machine; it is invoked by the service console. At normal run-time, the vmkernel is running on the bare computer, and the Linux-based service console runs as the first virtual machine. VMware dropped development of ESX at version 4.1, and now uses ESXi, which does not include a Linux kernel.
The vmkernel is a microkernel with three interfaces: hardware, guest systems, and the service console (Console OS).
Interface to hardware
The vmkernel handles CPU and memory directly, using scan-before-execution (SBE) to handle special or privileged CPU instructions and the SRAT (system resource allocation table) to track allocated memory.
Access to other hardware (such as network or storage devices) takes place using modules. At least some of the modules derive from modules used in the Linux kernel. To access these modules, an additional module called
vmklinux implements the Linux module interface. According to the README file, "This module contains the Linux emulation layer used by the vmkernel."
These drivers mostly equate to those described in VMware's hardware compatibility list. All these modules fall under the GPL. Programmers have adapted them to run with the vmkernel: VMware Inc has changed the module-loading and some other minor things.
In ESX (and not ESXi), the Service Console is a vestigial general purpose operating system most significantly used as bootstrap for the VMware kernel, vmkernel, and secondarily used as a management interface. Both of these Console Operating System functions are being deprecated from version 5.0, as VMware migrates exclusively to the ESXi model. The Service Console, for all intents and purposes, is the operating system used to interact with VMware ESX and the virtual machines that run on the server.
Purple Screen of Death
In the event of a hardware error, the vmkernel can catch a Machine Check Exception. This results in an error message displayed on a purple diagnostic screen. This is colloquially known as a purple diagnostic screen, or purple screen of death (PSoD, cf. Blue Screen of Death (BSoD)).
Upon displaying a purple diagnostic screen, the vmkernel writes debug information to the core dump partition. This information, together with the error codes displayed on the purple diagnostic screen can be used by VMware support to determine the cause of the problem.
VMware ESX is available in two main types: ESX and ESXi, although since version 5 only ESXi is continued.
ESX and ESXi before version 5.0 do not support Windows 8/Windows 2012. These Microsoft operating systems can only run on ESXi 5.x or later.
VMware ESXi, a smaller-footprint version of ESX, does not include the ESX Service Console. It is available - without the need to purchase a vCenter license - as a free download from VMware, with some features disabled.
VMware ESXi originated as a compact version of VMware ESX that allowed for a smaller 32 MB disk footprint on the host. With a simple configuration console for mostly network configuration and remote based VMware Infrastructure Client Interface, this allows for more resources to be dedicated to the guest environments.
Two variations of ESXi exist:
- VMware ESXi Installable
- VMware ESXi Embedded Edition
Originally named VMware ESX Server ESXi edition, through several revisions the ESXi product finally became VMware ESXi 3. New editions then followed: ESXi 3.5, ESXi 4, ESXi 5 and (as of 2015) ESXi 6.
GPL violation lawsuit
VMware has been sued by Christoph Hellwig, a Linux kernel developer, for GPL license violations. It was alleged that VMware had misappropriated portions of the Linux kernel, and used them without permission. The lawsuit was dismissed by the court in July 2016 and Hellwig announced he would file an appeal.
In the last stage of the lawsuit in March 2019, The Hamburg Higher Regional Court also rejected the claim on procedural grounds. Following this, VMware officially announced that they would remove the code in question. This followed with Hellwig withdrawing his case, and withholding further legal action.
Related or additional products
The following products operate in conjunction with ESX:
- vCenter Server, enables monitoring and management of multiple ESX, ESXi and GSX servers. In addition, users must install it to run infrastructure services such as:
- vMotion (transferring virtual machines between servers on the fly whilst they are running, with zero downtime)
- svMotion aka Storage vMotion (transferring virtual machines between Shared Storage LUNs on the fly, with zero downtime)
- Enhanced vMotion aka evMotion (a simultaneous vMotion and svMotion, supported on version 5.1 and above)
- Distributed Resource Scheduler (DRS) (automated vMotion based on host/VM load requirements/demands)
- High Availability (HA) (restarting of Virtual Machine Guest Operating Systems in the event of a physical ESX host failure)
- Fault Tolerance (FT) (almost instant stateful fail-over of a VM in the event of a physical host failure)
- Converter, enables users to create VMware ESX Server- or Workstation-compatible virtual machines from either physical machines or from virtual machines made by other virtualization products. Converter replaces the VMware "P2V Assistant" and "Importer" products — P2V Assistant allowed users to convert physical machines into virtual machines; and Importer allowed the import of virtual machines from other products into VMware Workstation.
- vSphere Client (formerly VMware Infrastructure Client), enables monitoring and management of a single instance of ESX or ESXi server. After ESX 4.1, vSphere Client was no longer available from the ESX/ESXi server, but must be downloaded from the VMware web site.
Cisco Nexus 1000v
Network-connectivity between ESX hosts and the VMs running on it relies on virtual NICs (inside the VM) and virtual switches. The latter exists in two versions: the 'standard' vSwitch allowing several VMs on a single ESX host to share a physical NIC and the 'distributed vSwitch' where the vSwitches on different ESX hosts together form one logical switch. Cisco offers in their Cisco Nexus product-line the Nexus 1000v, an advanced version of the standard distributed vSwitch. A Nexus 1000v consists of two parts: a supervisor module (VSM) and on each ESX host a virtual ethernet module (VEM). The VSM runs as a virtual appliance within the ESX cluster or on dedicated hardware (Nexus 1010 series) and the VEM runs as module on each host and replaces a standard dvS (distributed virtual switch) from VMware. Configuration of the switch is done on the VSM using the standard NX-OS CLI. It offers capabilities to create standard port-profiles which can then be assigned to virtual machines using vCenter.
There are several differences between the standard dvS and the N1000v; one is that the Cisco switch generally has full support for network technologies such as LACP link aggregation or that the VMware switch supports new features such as routing based on physical NIC load. However the main difference lies in the architecture: Nexus 1000v is working in the same way as a physical Ethernet switch does while dvS is relying on information from ESX. This has consequences for example in scalability where the Kappa limit for a N1000v is 2048 virtual ports against 60000 for a dvS. The Nexus1000v is developed in co-operation between Cisco and VMware and uses the API of the dvS
Third party management tools
Because VMware ESX is a leader in the server-virtualisation market, software and hardware vendors offer a range of tools to integrate their products or services with ESX. Examples are the products from Veeam Software with backup and management applications and a plugin to monitor and manage ESX using HP OpenView, Quest Software with a range of management and backup-applications and most major backup-solution providers have plugins or modules for ESX. Using Microsoft Operations Manager (SCOM) 2007/2012 with a Bridgeways ESX management pack gives you a realtime ESX datacenter health view.
VMware has added a Web Client since v5 but it will work on vCenter only and does not contain all features. vEMan is a Linux application which is trying to fill that gap. These are just a few examples: there are numerous 3rd party products to manage, monitor or backup ESX infrastructures and the VMs running on them.
Known limitations of VMware ESXi, as of Sept 2019, include the following:
- Guest system maximum RAM: 6128 GB
- Host system maximum RAM: 16 TB
- Number of hosts in a high availability or Distributed Resource Scheduler cluster: 64
- Maximum number of processors per virtual machine: 256
- Maximum number of processors per host: 768
- Maximum number of virtual CPUs per physical CPU core: 32
- Maximum number of virtual machines per host: 1024
- Maximum number of virtual CPUs per fault tolerant virtual machine: 4
- Maximum guest system RAM per fault tolerant virtual machine: 128 GB
- VMFS5 maximum volume size: 64 TB, but maximum file size is 62 TB -512 bytes
In terms of performance, virtualization imposes a cost in the additional work the CPU has to perform to virtualize the underlying hardware. Instructions that perform this extra work, and other activities that require virtualization, tend to lie in operating system calls. In an unmodified operating system, OS calls introduce the greatest portion of virtualization "overhead".
Paravirtualization or other virtualization techniques may help with these issues. VMware developed the Virtual Machine Interface for this purpose, and selected operating systems currently support this. A comparison between full virtualization and paravirtualization for the ESX Server shows that in some cases paravirtualization is much faster.
When using the advanced and extended network capabilities by using the Cisco Nexus 1000v distributed virtual switch the following network-related limitations apply:
- 64 ESX/ESXi hosts per VSM (Virtual Supervisor Module)
- 2048 virtual ethernet interfaces per VMware vDS (virtual distributed switch)
- and a maximum of 216 virtual interfaces per ESX/ESXi host
- 2048 active VLAN's (one to be used for communication between VEM's and VSM)
- 2048 port-profiles
- 32 physical NIC's per ESX/ESXi (physical) host
- 256 port-channels per VMware vDS (virtual distributed switch)
- and a maximum of 8 port-channels per ESX/ESXi host
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