OpenStack instances run on ephemeral disks—disks that only exist for the life of the instance. When an instance is terminated, the disk is discarded. This is a problem if there is any information that requires persistence. Block storage is one type of storage that can be used to persist data beyond the termination of an OpenStack instance.

Using Cinder, users can create block devices on demand and present them to running instances. The instances see this as a normal, everyday block device—as if an extra hard drive was plugged into the machine. The extra drive can be used as any other block device by creating partitions and file systems on it. Let's look now at how to create and present a block storage device using cinder.