Data written to currently running instances on disks is not persistent—when you terminate such instances, any disk writes will be lost. Volumes are persistent storage that you can attach to your running OpenStack compute instances; the best analogy is that of a USB drive that you can attach to an instance. Similar to USB drives, you can only attach instances to one computer at a time.

In prior OpenStack releases, volume services were provided by nova-volume, which has evolved over time into OpenStack Block Storage; that is, Cinder. OpenStack Block Storage is very similar to Amazon EC2's Elastic Block Storage—the difference is in how volumes are presented to the running instances. In OpenStack compute, volumes can easily be managed using an iSCSI-exposed LVM volume group named cinder-volumes. So, this iSCSI volume group must be present on any host running the Cinder volume service.

At times, managing OpenStack Block Storage can be confusing as Cinder volume is the running service name, and cinder-volumes is the name of the LVM volume group that is exposed by the cinder-volumes service.

In this chapter, we will be adding an additional node to run our OpenStack Block Storage services. The following diagram describes this environment and where Cinder fits: