These plugins dictate how a storage volumes (local files, local block devices, iSCSI LUNs, Ceph RBD devices etc etc) should be mapped to Virtual Machines running on Xen.

The xapi storage interface describes the concepts, features and APIs in more detail.

Datapath plugins are named by URI schemes. The following schemes are defined:

• raw+file: the block data is in a local file (or block device) and is in raw format
• vhd+file: the block data is in a vhd-formatted local file (or block device)

Internally we have the following low-level implementations. These should not be referenced directly by Volume plugins:

• loop+blkback: converts a file into a /dev/loop device using losetup and then connects the device to the VM with the blkback kernel driver.
• tapdisk: opens a file with tapdisk and then tapdisk serves the VM directly using the user-space grant-table and grant-mapping code.