An image is the foundation of VM instances in the cloud. An image is a single file that contains a virtual disk with a bootable operating system installed on it. The image provides a template for the creation of one or more VMs. Thus, to provision a VM on the cloud, you must first create an image.
Glance, the OpenStack image service, provides storage, discovery, registration, and delivery services for disk and server images. A registry server is an image service that provides image metadata information to clients. The image cache is used by the image service to obtain images on the local host rather than re-downloading them from the image server each time an image is requested.
You can upload multiple images to the Glance repository. As a best practice, upload images of different system types that you would deploy on the cloud. For example, create archived images of non-global zones, kernel zones, and global zones. Then you can deploy a VM of one of these types quickly by selecting the appropriate template.