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- in
the Xen scripts directory, commonly
prefix/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.