Images: The core idea of an image is that it is a bootable filesystem that contains an operating system. Images are the basos of instance (a virtual machine in OpenStack terminology). An image can be created from any of the following source files:
- raw - an unstructured disk image format.
- iso - An archive format for the data contents of an optical disc (e.g CDROM).
- qcow - a disk format supported by the QEMU emulator that can expand dynamically and supports Copy on Write.
These operating system images can be created and uploaded from your local computer or directly into the OpenStack deployment from an online source. Images are the primary source of an OS for creating instances (although volumes that contain a bootable filesystem can also be used instead).
When an image is used as the base OS for an instance a new copy is created and employed. The original image is not modified or locked and will not be deleted when the instance terminated. The image can, therefore, be used for other instances.
Instance Snapshot: A snapshot is an image which preserves the disk state of running instance. Snapshot are instantaneous copies of instances. They contain an image of the state of filesystem at the moment that the snapshot is taken. A snapshot of an instance can be used as the basis of an instance and booted up at a later time.
Volume: A volume is a block of storage device. Volumes can be attached to instances and will appears as a new block device. In this way, they can be though of as a spare hard disk that can be attached to instances and used as an independent storage device.