Hypervisors????

A hypervisor or virtual machine monitor (VMM) is computer software, firmware, or hardware, that creates and runs virtual machines. A computer on which a hypervisor runs one or more virtual machines is called a host machine, and each virtual machine is called a guest machine. The hypervisor presents the guest operating systems with a virtual operating platform and manages the execution of the guest operating systems. Multiple instances of a variety of operating systems may share the virtualized hardware resources: for example, Linux, Windows, and OS X instances can all run on a single physical x86 machine. This contrasts with operating-system-level virtualization, where all instances (usually called containers) must share a single kernel, though the guest operating systems can differ in user space, such as different Linux distributions with the same kernel.
The term hypervisor is a variant of supervisor, a traditional term for the kernel of an operating system: the hypervisor is the supervisor of the supervisor, with hyper- used as a stronger variant of super. The term dates to circa 1970, in the earlier CP/CMS (1967) system the term Control Program was used instead.

Classification

Type-1 and type-2 hypervisors

Type-1, native or bare-metal hypervisors

These hypervisors run directly on the host's hardware to control the hardware and to manage guest operating systems. For this reason, they are sometimes called bare metal hypervisors. The first hypervisors, which IBM developed in the 1960s, were native hypervisors. These included the test software SIMMON and the CP/CMS operating system (the predecessor of IBM's z/VM). Modern equivalents include Xen, Oracle VM Server for SPARC, Oracle VM Server for x86, Microsoft Hyper-V and VMware ESX/ESXi.

Type-2 or hosted hypervisors

These hypervisors run on a conventional operating system (OS) just as other computer programs do. A guest operating system runs as a process on the host. Type-2 hypervisors abstract guest operating systems from the host operating system. VMware Workstation, VMware Player, VirtualBox, Parallels Desktop for Mac and QEMU are examples of type-2 hypervisors.

However, the distinction between these two types is not necessarily clear. Linux's Kernel-based Virtual Machine (KVM) and FreeBSD's bhyve are kernel modules that effectively convert the host operating system to a type-1 hypervisor. At the same time, since Linux distributions and FreeBSD are still general-purpose operating systems, with other applications competing for VM resources, KVM and bhyve can also be categorized as type-2 hypervisors.

The modern hypervisor: A high-level explanation

The evolution of virtualization greatly revolves around one piece of very important software. This is the hypervisor. As an integral component, this software piece allows for physical devices to share their resources amongst virtual machines running as guests on to top of that physical hardware. To further clarify the technology, it’s important to analyze a few key definitions:

• Type I Hypervisor. This type of hypervisor (pictured at the beginning of the article) is deployed as a bare-metal installation. This means that the first thing to be installed on a server as the operating system will be the hypervisor. The benefit of this software is that the hypervisor will communicate directly with the underlying physical server hardware. Those resources are then paravirtualized and delivered to the running VMs. This is the preferred method for many production systems.
• Type II Hypervisor. This model (shown below) is also known as a hosted hypervisor. The software is not installed onto the bare-metal, but instead is loaded on top of an already live operating system. For example, a server running Windows Server 2008R2 can have VMware Workstation 8 installed on top of that OS. Although there is an extra hop for the resources to take when they pass through to the VM – the latency is minimal and with today’s modern software enhancements, the hypervisor can still perform optimally.
• Guest Machine. A guest machine, also known as a virtual machine (VM) is the workload installed on top of the hypervisor. This can be a virtual appliance, operating system or other type of virtualization-ready workload. This guest machine will, for all intents and purposes, believe that it is its own unit with its own dedicated resources. So, instead of using a physical server for just one purpose, virtualization allows for multiple VMs to run on top of that physical host. All of this happens while resources are intelligently shared between other VMs.
• Host Machine.  This is known as the physical host. Within virtualization, there may be several components – SAN, LAN, wiring, and so on. In this case, we are focusing on the resources located on the physical server. The resource can include RAM and CPU. These are then divided between VMs and distributed as the administrator sees fit. So, a machine needing more RAM (a domain controller) would receive that allocation, while a less important VM (a licensing server for example) would have fewer resources. With today’s hypervisor technologies, many of these resources can be dynamically allocated.
• Paravirtualization Tools. After the guest VM is installed on top of the hypervisor, there usually is a set of tools which are installed into the guest VM. These tools provide a set of operations and drivers for the guest VM to run more optimally. For example, although natively installed drivers for a NIC will work, paravirtualized NIC drivers will communicate with the underlying physical layer much more efficiently. Furthermore, advanced networking configurations become a reality when paravirtualized NIC drivers are deployed.

Now that there is a better understanding of the hypervisor and the various components which fall under it, we can examine the major players in the industry.

The “Big Three” Hypervisor Vendors

Although there are several smaller organizations which are developing their own hypervisor technologies, three manufacturers have really taken the market by storm with their solutions. As leaders in the space, there are specific differentiators between the products. This doesn’t mean one is necessarily better than the other;  rather, it means that there may be a better fit for one hypervisor over another.
VMware vSphere 5 – VMware has come a long way in the hypervisor market. It still commands the top spot within the server virtualization world and makes some of the best application and desktop virtualization technologies. As VMware continues to evolve, the product is known for its feature-rich suite capable of some very powerful solutions. Integration with DR, SAN, LAN and WAN technologies makes VMware an intricate component to many modern data center environments. One of the biggest challenges facing a potential customer is understanding which feature and license set is required for a given project. Prior to any purchase, the IT team should plan out the deployment and have a clear picture to which features are key for the environment.
Citrix XenServer 6 – As a leader in the application virtualization and delivery markets, it was only a matter of time before a hypervisor came into the picture. Originally known as XenSource, XenServer was born from an open source world and built upon enterprise technologies. Development of the XenServer platform has come a long way. Now, more organizations are deploying this technology into their production and test systems which are capable of handling a global load. The entry price point for the hypervisor is very enticing and the Enterprise version contains many of the necessary enterprise features which administrators demand. Failover, HA, shared resources and other key components are all native to the hypervisor and are ready for production rollouts. Arguably second in the server virtualization space, XenServer continues to innovate and expand its product offering.
Microsoft Hyper-V 3 – With the latest release of Windows Server just around the corner, there has been a lot of excited conversation revolving around the latest iteration of Microsoft’s Hyper-V. Already a solid platform, some limitations are being directly addressed with the new released. Live migration, storage resource pools and even cloud backup capabilities are all being built into the new hypervisor technology. Couple that with the very low price point to purchase – and we may very well see a new powerhouse emerging. The newest version of Hyper-V looks to take some serious market-share away from both Citrix and VMware as it deploys a more production and enterprise ready hypervisor. The direct integration with Windows Server systems will make this product even more enticing.

Although VMware is currently the market leader within the virtualization space, others are quickly emerging as leaders in their space. Remember, in this article, we’ve mainly been discussing server virtualization. As industry demands have grown, technologies have expanded beyond the server. Now, there are technologies for application virtualization (ThinApp and XenApp) as well as technologies which can virtualize desktops (XenDesktop and View). The entire idea behind a virtual data center is to create a more efficient environment which is easier to manage and orchestrate. Furthermore, concepts such as DR and physical server consolidation are all made easier when virtualization is introduced. There is very little doubt that the technology will continue to be adopted and improved. The exciting part is watching how further innovations can help organizations of all sizes align their business goals with their IT infrastructure.

What are hypervisors used for?

Hypervisors are important to any system administrator or system operator because virtualization adds a crucial layer of management and control over the data center and enterprise environment. Staff members not only need to understand how the respective hypervisor works, but also how to operate supporting functionality such as VM configuration, migration and snapshots.

The role of a hypervisor is also expanding. For example, storage hypervisors are used to virtualize all of the storage resources in the environment to create centralized storage pools that administrators can provision -- without having to concern themselves with where the storage was physically located. Today, storage hypervisors are a key element of software-defined storage. Networks are also being virtualized with hypervisors, allowing networks and network devices to be created, changed, managed and destroyed entirely through software without ever touching physical network devices. As with storage, network virtualization is appearing in broader software-defined network or software-defined data center platforms.