Ephemeral Disks can most easy be seen as the equivalent of local internal disks in an ordinary computer.
When the Server Flavours in the Compute service are configured to provide instances with root or ephemeral disks, the compute service manages these allocations using its ephemeral disk storage location.
In many environments, the ephemeral disks are stored on the Compute host’s local disks, but for production environments we recommend that the Compute hosts be configured to use a shared storage subsystem instead.
A shared storage subsystem allows quick, live instance migration between Compute hosts, which is useful when an administrator needs to perform maintenance on the Compute host and wants to evacuate it. Using a shared storage subsystem also allows the recovery of instances when a Compute host goes offline. The administrator is able to evacuate the instance to another Compute host and boot it up again. The following diagram illustrates the interactions between the storage device, the Compute host, the hypervisor, and the instance.
The diagram shows the following steps.
|1||The Compute host is configured with access to the storage device. The Compute host accesses the storage space via the storage network by using a storage protocol (for example, NFS, iSCSI, or Ceph RBD).|
|2||The nova-compute service configures the hypervisor to present the allocated instance disk as a device to the instance.|
|3||The hypervisor presents the disk as a device to the instance.|
Mainly taken from OpenStack.org