Hypervisor selection

The majority of installations use either the Xen or KVM hypervisors, but there are a number of other hypervisors available for use with Nova. Both the Hyper-V and VMware hypervisors are supported as compute platforms. Bare-metal systems can be provisioned through Nova (Ironic project) and various drivers for containers have come and gone over the life of OpenStack. Running containers in OpenStack is a topic of great interest lately, and the OpenStack Magnum project has emerged as the current favorite model for their deployment within OpenStack.

Whether to pick the Xen or KVM hypervisor is almost a religious matter for some people. The KVM hypervisor is the most broadly used hypervisor on Linux, and it has support for a wide range of advanced features, such as PCI-passthrough, NUMA zone pinning, and live migration. Proponents of Xen will point out that performance on older hardware has traditionally been better with Xen. They will also point out that large cloud providers such as Amazon and Rackspace have picked the Xen hypervisor over the KVM hypervisor for their deployments. However, adoption continues to wane for the Xen hypervisor. Based on the 2017 OpenStack User Survey report, 86% of OpenStack clouds in production are using KVM and only 11% are using Xen. KVM is the default and most widely used hypervisor, and unless your organization has a strong motivation to pick the Xen hypervisor, we strongly recommend that you use it over any other hypervisor.

A number of good arguments can be made for using VMware ESX as the hypervisor in OpenStack instead of KVM. Almost every large IT shop has a great deal of experience with ESX and vSphere, and using ESX as the hypervisor for an OpenStack deployment can ease the barrier for entry to OpenStack for teams that don't want to certify or train up on a new virtualization technology. ESX-based deployments are prescriptive; there is only one Cinder driver and one Neutron driver supported with ESX. From this perspective, choosing ESX allows an organization to focus on the integration and operational aspects of OpenStack without worrying too much about the architecture.

The same reasons that make ESX a good choice for some organizations make it a poor choice for many organizations, though. The prescriptive approach limits available options in the storage and network space. One of the strongest aspects of OpenStack is the heterogeneity of the ecosystem and the ability to dynamically add new types of infrastructure under a common API. For this reason, we'll be focusing our work in this book on the use of the KVM hypervisor. Most of the considerations we address in this section will also apply to the Xen hypervisor.