OpenStack provides two classes of block storage, "ephemeral" storage and persistent "volumes". Ephemeral storage exists only for the life of an instance, it will persist across reboots of the guest operating system but when the instance is deleted so is the associated storage. All instances have some ephemeral storage. Volumes are persistent virtualized block devices independent of any particular instance. Volumes may be attached to a single instance at a time, but may be detached or reattached to a different instance while retaining all data, much like a USB drive.
Ephemeral storage is associated with a single unique instance. Its size is defined by the flavor of the instance.
Data on ephemeral storage ceases to exist when the instance it is associated with is terminated. Rebooting the VM or restarting the host server, however, will not destroy ephemeral data. In the typical use case an instance's root filesystem is stored on ephemeral storage. This is often an unpleasant surprise for people unfamiliar with the cloud model of computing.
In addition to the ephemeral root volume all flavors except the smallest, m1.tiny, provide an additional ephemeral block device varying from 20G for the m1.small through 160G for the m1.xlarge by default - these sizes are configurable. This is presented as a raw block device with no partition table or filesystem. Cloud aware operating system images may discover, format, and mount this device. For example the cloud-init package included in Ubuntu's stock cloud images will format this space as an ext3 filesystem and mount it on /mnt. It is important to note this a feature of the guest operating system. OpenStack only provisions the raw storage.
Volume storage is independent of any particular instance and is persistent. Volumes are user created and within quota and availability limits may be of any arbitrary size.
When first created volumes are raw block devices with no partition table and no filesystem. They must be attached to an instance to be partitioned and/or formatted. Once this is done they may be used much like an external disk drive. Volumes may attached to only one instance at a time, but may be detached and reattached to either the same or different instances.
It is possible to configure a volume so that it is bootable and provides a persistent virtual instance similar to traditional non-cloud based virtualization systems. In this use case the resulting instance may still have ephemeral storage depending on the flavor selected, but the root filesystem (and possibly others) will be on the persistent volume and thus state will be maintained even if the instance it shutdown. Details of this configuration are discussed in the Boot From Volume section of this manual.
Volumes do not provide concurrent access from multiple instances. For that you need either a traditional network filesystem like NFS or CIFS or a cluster filesystem such as GlusterFS. These may be built within an OpenStack cluster or provisioned outside of it, but are not features provided by the OpenStack software.
