Instance boot source

An instance boots from either an image or a bootable volume. Image-backed instance disks are ephemeral and deleted with the instance. Volume-backed instance disks persist independently.

An image contains a bootable operating system. The Image service (glance) controls image storage and management. You can launch any number of instances from the same base image. Each instance runs from a copy of the base image. Any changes that you make to the instance do not affect the base image.

A bootable volume is a block storage volume created from an image that contains a bootable operating system. The instance can use the bootable volume to persist instance data when the instance is deleted. You can use an existing persistent root volume when you launch an instance. You can also create persistent storage when you launch an instance from an image, so that you can save the instance data when the instance is deleted. A new persistent storage volume is created automatically when you create an instance from a volume snapshot.

The following diagram shows the instance disks and storage that you can create when you launch an instance. The actual instance disks and storage created depend on the boot source and flavor used.