code: def _initDMinfo(self):
          
          self._mapLVtuple2dm = {}
          self._mapLVname2dm = {}
          self._vgTree = {}
          if self._dmMajorNum is None:
              self._initBlockMajorMap()
          for file in os.listdir(devmapperDir):
              mobj = re.match('([a-zA-Z0-9+_.\-]*[a-zA-Z0-9+_.])-([a-zA-Z0-9+_.][a-zA-Z0-9+_.\-]*)$', file)
              if mobj:
                  path = os.path.join(devmapperDir, file)
                  (major, minor) = self._getDevMajorMinor(path)
                  if major == self._dmMajorNum:
                      vg = mobj.group(1).replace('--', '-')
                      lv = mobj.group(2).replace('--', '-')
                      dmdev = "dm-%d" % minor
                      self._mapLVtuple2dm[(vg,lv)] = dmdev
                      self._mapLVname2dm[file] = dmdev
                      if not vg in self._vgTree:
                          self._vgTree[vg] = []
                      self._vgTree[vg].append(lv)
docs: Check files in /dev/mapper to initialize data structures for 
          mappings between device-mapper devices, minor device numbers, VGs 
          and LVs.