What is Glance?

Glance is an image service that allows users to discover, retrieve, and register VM (virtual machine) images and container images, which can use Swift or Ceph as its actual storage backend (i.e. not general-purpose object storage). Currently, Glance is the only supported image service. By default, Compute is configured to use it. Glance can be used as an image registry for sharing images with anyone that has access to it, meaning that it can allow sharing images of critical components to suit your business needs.

The OpenStack Image service (Glance) includes the following components:

  • Glance-api – Accepts Image API calls for image discovery, retrieval, and storage.
  • Glance-registry – Stores, processes, and retrieves metadata about images. Metadata includes items such as size and type.
  • Database – Stores image metadata and you can choose your database depending on your preference. Most deployments use MySQL or SQLite.
  • Storage repository for image files – Various repository types are supported including normal file systems (or any filesystem mounted on the glance-api controller node), Object Storage, RADOS block devices, VMware datastore, and HTTP. Note that some repositories will only support read-only usage.
  • Metadata definition service – A common API for vendors, admins, services, and users to meaningfully define their own custom metadata. This metadata can be used on different types of resources like images, artifacts, volumes, flavors, and aggregates. A definition includes the new property’s key, description, constraints, and the resource types which it can be associated with.

You can find more information and even contribute to the OpenStack Glance documentation site.

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