OpenStack Glance is an image repository that stores virtual disk images. Users can upload new images and also take a snapshot of a running virtual machine and store it as an image. The snapshot can then be used as a template to launch new virtual machines. Glance uses a database to store the images and its metadata.

Glance supports the following image formats:

• AMI (Amazon Machine Image)
• AKI (Amazon Kernel Image)
• ARI (Amazon Ram Disk)
• ISO (archive format for CDs)
• qcow2 (QEMU/KVM image format)
• raw (raw unstructured format)
• vhd (Hyper-V format )
• vhdx (enhanced vhd format)
• vdi (QEMU/VirtualBox format)
• vmdk (VMware format)

Glance also supports the following container formats:

• bare (no metadata)
• ova (OVA tar archive)
• ovf 
• aki/ami/ari (Amazon Machine Image/Amazon Kernel Image/Amazon Ramdisk Image)
• docker (docker tar archive)

For developers, OpenStack Glance provides a RESTful API that allows querying of virtual disk images and their metadata. A Glance domain controller manages tasks that are divided into separate layers.

The glance-store library interacts with the storage backends or local filesystem. It also uses a central database called Glance DB which is shared between all other components. 

The following diagram shows the various components of the OpenStack Glance service:

As shown in the diagram, the client invokes REST APIs that get authenticated from Keystone. The image can be fetched either from the GlanceDB or from an external storage using the Glance Store Drivers.

The default installation of OpenStack provides a tiny Linux image called Cirros in the Glance database. The following screenshot shows the OpenStack Glance user interface and the default Cirros image provided.  

Lets now move on to the OpenStack block storage service.