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Image service overview
The Image service (glance) enables users to discover, register, and
retrieve virtual machine images. It offers a REST <RESTful> API
that enables you to query virtual machine image metadata and retrieve an
actual image. You can store virtual machine images made available
through the Image service in a variety of locations, from simple file
systems to object-storage systems like OpenStack Object Storage.
Important
For simplicity, this guide describes configuring the Image service to
use the file back end, which uploads and stores in a
directory on the controller node hosting the Image service. By default,
this directory is /var/lib/glance/images/.
Before you proceed, ensure that the controller node has at least
several gigabytes of space available in this directory. Keep in mind
that since the file back end is often local to a controller
node, it is not typically suitable for a multi-node glance
deployment.
For information on requirements for other back ends, see Configuration Reference.
The OpenStack Image service is central to Infrastructure-as-a-Service
(IaaS) as shown in get_started_conceptual_architecture. It accepts API
requests for disk or server images, and metadata definitions from end
users or OpenStack Compute components. It also supports the storage of
disk or server images on various repository types, including OpenStack
Object Storage.
A number of periodic processes run on the OpenStack Image service to support caching. Replication services ensure consistency and availability through the cluster. Other periodic processes include auditors, updaters, and reapers.
The OpenStack Image service includes the following components:
- glance-api
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Accepts Image API calls for image discovery, retrieval, and storage.
- glance-registry
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Stores, processes, and retrieves metadata about images. Metadata includes items such as size and type.
Warning
The registry is a private internal service meant for use by OpenStack Image service. Do not expose this service to users.
- 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
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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
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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.