The udev rules included in lvm2 are responsible for setting up the devices
in /dev/$VG and also provides some environment variables for use by later
rules:

* DM_VG_NAME
  Volume group name
* DM_LV_NAME
  Logical volume name
* DM_LV_LAYER
  Logical volume layer (blank if not set)

The rules included in dmsetup are responsible for the core device mapper
support, see /usr/share/doc/dmsetup/README.udev for further information.
