Chapter 4. Compute Nodes

In this chapter, we discuss some of the choices you need to consider when building out your compute nodes. Compute nodes form the resource core of the OpenStack Compute cloud, providing the processing, memory, network and storage resources to run instances.

 Choosing a Hypervisor

A hypervisor provides software to manage virtual machine access to the underlying hardware. The hypervisor creates, manages, and monitors virtual machines. OpenStack Compute supports many hypervisors to various degrees, including:

Probably the most important factor in your choice of hypervisor is your current usage or experience. Aside from that, there are practical concerns to do with feature parity, documentation, and the level of community experience.

For example, KVM is the most widely adopted hypervisor in the OpenStack community. Besides KVM, more deployments run Xen, LXC, VMware, and Hyper-V than the others listed. However, each of these are lacking some feature support or the documentation on how to use them with OpenStack is out of date.

The best information available to support your choice is found on the Hypervisor Support Matrix and in the configuration reference.


It is also possible to run multiple hypervisors in a single deployment using host aggregates or cells. However, an individual compute node can run only a single hypervisor at a time.

 Instance Storage Solutions

As part of the procurement for a compute cluster, you must specify some storage for the disk on which the instantiated instance runs. There are three main approaches to providing this temporary-style storage, and it is important to understand the implications of the choice.

They are:

In general, the questions you should ask when selecting storage are as follows:

Many operators use separate compute and storage hosts. Compute services and storage services have different requirements, and compute hosts typically require more CPU and RAM than storage hosts. Therefore, for a fixed budget, it makes sense to have different configurations for your compute nodes and your storage nodes. Compute nodes will be invested in CPU and RAM, and storage nodes will be invested in block storage.

However, if you are more restricted in the number of physical hosts you have available for creating your cloud and you want to be able to dedicate as many of your hosts as possible to running instances, it makes sense to run compute and storage on the same machines.

We'll discuss the three main approaches to instance storage in the next few sections.

 On Compute Node Storage—Shared File System

In this option, each compute node is specified with a significant amount of disk space, but a distributed file system ties the disks from each compute node into a single mount.

The main advantage of this option is that it scales to external storage when you require additional storage.

However, this option has several downsides:

  • Running a distributed file system can make you lose your data locality compared with nonshared storage.

  • Recovery of instances is complicated by depending on multiple hosts.

  • The chassis size of the compute node can limit the number of spindles able to be used in a compute node.

  • Use of the network can decrease performance.

 Issues with Live Migration

We consider live migration an integral part of the operations of the cloud. This feature provides the ability to seamlessly move instances from one physical host to another, a necessity for performing upgrades that require reboots of the compute hosts, but only works well with shared storage.

Live migration can also be done with nonshared storage, using a feature known as KVM live block migration. While an earlier implementation of block-based migration in KVM and QEMU was considered unreliable, there is a newer, more reliable implementation of block-based live migration as of QEMU 1.4 and libvirt 1.0.2 that is also compatible with OpenStack. However, none of the authors of this guide have first-hand experience using live block migration.


OpenStack allows you to overcommit CPU and RAM on compute nodes. This allows you to increase the number of instances you can have running on your cloud, at the cost of reducing the performance of the instances. OpenStack Compute uses the following ratios by default:

The default CPU allocation ratio of 16:1 means that the scheduler allocates up to 16 virtual cores per physical core. For example, if a physical node has 12 cores, the scheduler sees 192 available virtual cores. With typical flavor definitions of 4 virtual cores per instance, this ratio would provide 48 instances on a physical node.

The formula for the number of virtual instances on a compute node is (OR*PC)/VC, where:

Similarly, the default RAM allocation ratio of 1.5:1 means that the scheduler allocates instances to a physical node as long as the total amount of RAM associated with the instances is less than 1.5 times the amount of RAM available on the physical node.

For example, if a physical node has 48 GB of RAM, the scheduler allocates instances to that node until the sum of the RAM associated with the instances reaches 72 GB (such as nine instances, in the case where each instance has 8 GB of RAM).


Regardless of the overcommit ratio, an instance can not be placed on any physical node with fewer raw (pre-overcommit) resources than the instance flavor requires.

You must select the appropriate CPU and RAM allocation ratio for your particular use case.


Networking in OpenStack is a complex, multifaceted challenge. See Chapter 7, Network Design.


Compute nodes are the workhorse of your cloud and the place where your users' applications will run. They are likely to be affected by your decisions on what to deploy and how you deploy it. Their requirements should be reflected in the choices you make.

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