1.4 OpenStack Instances

OpenStack virtual machines are called instances, mostly because they are instances of an image that is created upon request and that is configured when launched. The main difference between OpenStack and traditional virtualization technology is the way state is stored. With traditional virtualization technology, the state of the virtual machine is persistent.

OpenStack can support both persistent and ephemeral models. In the ephemeral model, an instance is launched from an image in the Image service, the image is copied to the run area, and when the copy is completed, the instance starts running. The size and connectivity of the instance are defined at the time of launching the instance. When an instance is terminated, the original image remains intact, but the state of the terminated instance is not retained. This ephemeral model is useful for scaling out quickly and maintaining agility for users.

In the persistent model, the instance is launched from a persistent volume on a compute node, or from a block storage volume, and not from the Image service. A volume can be any kind of persistent storage, including a file, a block device, an LVM partition, or any other form of persistent storage. In this case, when the instance is terminated, any session changes are retained and are present the next time an instance is launched. In the persistent model, the size and connectivity of the instance are also defined at the time the instance launches. In some sense, the persistent model in OpenStack is similar to the traditional approach to virtualization.