Glance - Image registry

The Glance service provides a registry of images and metadata that the OpenStack user can launch as a virtual machine. Various image formats are supported and can be used based on the choice of hypervisor. Glance supports images for KVM/Qemu, XEN, VMware, Docker, and so on.

As a new user of OpenStack, one might often wonder, What is the difference between Glance and Swift? Both handle storage. What is the difference between them? Why do I need to integrate such a solution?

Swift is a storage system, whereas Glance is an image registry. The difference between the two is that Glance is a service that keeps track of virtual machine images and metadata associated with the images. Metadata can be information such as a kernel, disk images, disk format, and so on. Glance makes this information available to OpenStack users over REST APIs. Glance can use a variety of backends for storing images. The default is to use directories, but in a massive production environment it can use other approaches such as NFS and even Swift.

Swift, on the other hand, is a storage system. It is designed for object-storage where you can keep data such as virtual disks, images, backup archiving, and so on.

The mission of Glance is to be an image registry. From an architectural point of view, the goal of Glance is to focus on advanced ways to store and query image information via the Image Service API. A typical use case for Glance is to allow a client (which can be a user or an external service) to register a new virtual disk image, while a storage system focuses on providing a highly scalable and redundant data store. At this level, as a technical operator, your challenge is to provide the right storage solution to meet cost and performance requirements. This will be discussed at the end of the book.