Cinder is the block storage component in OpenStack. In the previous chapter, all the necessary resources were collected to launch an instance. Now that this instance is running, let's look at the use case for block storage and the process of attaching virtual block storage to the OpenStack instance. Then, we will take a look at the storage engine used to store these block devices and the other options available for the backing store.
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 filesystems on it. Let's look now at how to create and present a block storage device using Cinder.