Nova-Volume

Concept:

• is the service that allows you to give extra block level storage to your OpenStack Compute instances.
• is an iSCSI solution that employs the use of Logical Volume Manager (LVM) for Linux.
• is not a ‘shared storage’ solution like a SAN of NFS on which multiple servers can attach to.

The nova-volumes service uses iSCSI-exposed LVM volumes to the compute nodes which run instances. Thus, there are two components involved:

1. lvm2, which works with a VG called "nova-volumes" (Refer to http://en.wikipedia.org/wiki/Logical_Volume_Manager_(Linux) for further details)
2. open-iscsi, the iSCSI implementation which manages iSCSI sessions on the compute nodes

Here is what happens from the volume creation to its attachment (we use euca2ools for examples, but the same explanation goes with the API):

1. The volume is created via $euca-create-volume; which creates an LV into the volume group (VG) "nova-volumes"
2. The volume is attached to an instance via $euca-attach-volume; which creates a unique iSCSI IQN that will be exposed to the compute node.
3. The compute node which run the concerned instance has now an active ISCSI session; and a new local storage (usually a /dev/sdX disk)

     4.  libvirt uses that local storage as a storage for the instance; the instance get a new disk (usually a /dev/vdX disk).