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Chapter 1. About instances
Instances are the individual virtual machines that run on physical Compute nodes inside the cloud. To launch an instance, you need a flavor and either an image or a bootable volume. When you use an image to launch an instance, the provided image becomes the base image that contains a virtual disk installed with a bootable operating system. Each instance requires a root disk, which we refer to as the instance disk. The Compute service (nova) resizes the instance disk to match the specifications of the flavor that you specified for the instance.
Images are managed by the Image Service (glance). The Image Service image store contains a number of predefined images. The Compute nodes provide the available vCPU, memory, and local disk resources for instances. The Block Storage service (cinder) provides predefined volumes. Instance disk data is stored either in ephemeral storage, which is deleted when you delete the instance, or in a persistent volume provided by the Block Storage service.
The Compute service is the central component that provides instances on demand. The Compute service creates, schedules, and manages instances, and interacts with the Identity service for authentication, the Image service for the images used to launch instances, and the Dashboard service (horizon) for the user and administrative interface. As a cloud user, you interact with the Compute service when you create and manage your instances. You can create and manage your instances by using the OpenStack CLI or the Dashboard.