bare-metal hypervisor (Type 1 hypervisor)

What is a bare-metal hypervisor (Type 1 hypervisor)?

A bare-metal hypervisor, also known as a Type 1 hypervisor, is virtualization software that has been installed directly onto the computing hardware. This type of hypervisor controls not only the hardware but also one or more guest operating systems (guest OSes).

In comparison, a hosted hypervisor, or Type 2 hypervisor, runs on the host OS. The hypervisor communicates with the host OS, which communicates with the underlying hardware. The host OS controls the hardware, not the hypervisor, which adds another layer of abstraction to the software stack, increasing latency in the virtual machines (VMs).

Because bare-metal hypervisors access the hardware directly, their VMs perform better than those running on hosted hypervisors -- an important consideration when deploying mission-critical applications. Bare-metal hypervisors also offer greater scalability, stability and availability, while providing more robust resource management features.

What is the difference between bare-metal and hosted hypervisors?

A bare-metal hypervisor is significantly different from a hosted hypervisor. Although both are capable of hosting multiple VMs, a Type 2 hypervisor runs on top of a parent OS, or the host OS, whereas a bare-metal hypervisor is installed directly onto the server hardware. This difference leads to several other key distinctions.

One of the biggest lies in the way that VMs consume hardware resources. Because a bare-metal hypervisor is installed directly on the hardware, a VM can access the hardware resources directly through the virtualization layer.

Conversely, a VM that is running on a hosted hypervisor must pass hardware requests through virtualization layer and then to the host OS, which represents an additional layer of software abstraction. A bare-metal hypervisor avoids this extra layer, resulting in better VM performance.

A bare-metal hypervisor also offers better security than a hosted hypervisor. Because a hosted hypervisor is dependent on the underlying OS, security vulnerabilities within that OS could potentially be exploited to get at the guest OSes running within the VMs.

A Type 1 hypervisor runs on bare metal, and a Type 2 hypervisor runs on top of an operating system.

What are the benefits of bare-metal hypervisors?

Type 1 hypervisors are used extensively in enterprise and cloud data centers because of the many benefits they offer over Type 2 hypervisors:

• Performance. Because a Type 1 hypervisor runs on bare metal, the VMs can operate at near-native hardware speeds.
• Security. Type 1 VMs are not susceptible to vulnerabilities, such as a virtual machine escape attack, that might exist in the underlying OS, as can be the case for a hosted hypervisor.
• Scalability. Enterprise grade bare-metal hypervisors support the creation of large failover clusters and can generally take full advantage of the server's hardware resources, such as CPU and memory.
• Manageability. Bare-metal hypervisor vendors offer management consoles that enable administrators to manage all their VMs through a single interface.

For enterprise and cloud environments, Type 1 hypervisors are typically preferred over Type 2. Bare-metal hypervisors also offer several advantages over traditional server deployments, in which workloads also run on bare metal, but they're not virtualized.

Virtualization in general can help organizations utilize resources more efficiently while strengthening security through VM isolation. It also offers greater flexibility and increases scalability. In addition, virtualization makes it easier to manage infrastructure and server environments and implement disaster recovery strategies while reducing costs and energy consumption.

What are the drawbacks of bare-metal hypervisors?

Although Type 1 hypervisors offer several benefits, they are not without their challenges:

• Cost. Bare-metal hypervisors can be significantly more expensive than hosted hypervisors. They also require dedicated hardware.
• Complexity. Enterprise class bare-metal hypervisors can be difficult to deploy and manage. IT teams might face a substantial learning curve associated with their use.
• Driver limitations. The hypervisor's built-in device drivers can limit hardware support. However, this is usually not much of an issue when working with standard hardware and well-known Type 1 hypervisors such as Microsoft Hyper-V, Citrix XenServer or VMware ESXi.

Type 1 hypervisors also have drawbacks when compared to traditional bare-metal server deployments. For example, many IT teams face the challenge of VM sprawl, in which more VMs are created than needed. The VMs might also be left running and forgotten, resulting in wasted resources and potential security and regulatory compliance risks.

To make matters worse, it can be difficult for administrators to monitor and track all their VMs as well as ensure each one has the resources it needs to support its workloads. IT teams must also ensure the software running in their VMs is compliant with applicable licensing requirements. In addition, they might find that some software is not suited to a virtual environment, although this issue has become less common in recent years.

When should an organization use a bare-metal hypervisor?

Bare-metal hypervisors are best suited for organizations that require a high level of performance, scalability, management and security:

• Performance. Because a bare-metal hypervisor is installed directly on the server hardware, the VMs run at speeds nearly comparable to non-virtualized workloads. In contrast, a Type 2 hypervisor runs on top of the host OS, adding an additional abstraction layer, which can impact performance.
• Scalability. Bare-metal hypervisors tend to be more scalable than hosted hypervisors, in large part because they're not installed on the host OS. A Type 2 hypervisor is limited by the underlying OS. If the OS doesn't allow clustering or supports only a relatively small amount of memory, the hypervisor is subject to those limitations.
• Management. Bare-metal hypervisor vendors generally offer a management console that is designed to support large-scale hypervisor deployments. For example, VMware provides vCenter Server, and Microsoft offers System Center Virtual Machine Manager as part of Microsoft System Center. Such consoles help to simplify the process of managing large-scale VM deployments.
• Security. Because a bare-metal hypervisor does not run on top of an OS, it is inherently more secure than a hosted hypervisor, which can be susceptible to security vulnerabilities that exist within the host OS.

Bare-metal virtualization is ideally suited to large organizations or to those organizations with significant performance or security requirements. Hosted hypervisors tend to be a better fit for development, testing, research or SMB environments.

What are the top bare-metal hypervisors?

There are several popular bare-metal hypervisors, both commercial and open source. Here are some examples of well-known bare-metal hypervisor products:

• Citrix XenServer, formerly Citrix Hypervisor.
• Kernel-based Virtual Machine (KVM).
• Microsoft Hyper-V.
• Nutanix AHV.
• Oracle VM Server for x86.
• RedHat Virtualization.
• VMware ESXi.
• Xen Project.

Learn more about bare-metal hypervisors in this beginner's guide to hosted and bare-metal virtualization. Compare Nutanix AHV vs. VMware ESXi, explore the differences between Xen vs. KVM and see how KVM stacks up against Microsoft Hyper-V.