Xen supports two Virtual Block Device types natively:

phy. This device type is used to hand "physical" block devices, available in the host environment, off to a guest domU in an essentially transparent fashion.

file. This device type is used to make file-based block device images available to the guest domU. It works by creating a loop block device from the original image file, and then handing that block device off to the domU in much the same fashion as the phy device type does.

If a Virtual Block Device configured in the disk option of a domU configuration uses any prefix other than phy:, file:, or no prefix at all (in which case Xen defaults to using the phy device type), Xen expects to find a helper script named block-prefix in the Xen scripts directory, commonly /etc/xen/scripts.

The DRBD distribution provides such a script for the drbd device type, named /etc/xen/scripts/block-drbd. This script handles the necessary DRBD resource state transitions as described earlier in this chapter.