code: def _sectors(self, ignore_chunk=None):
          
          sectorsize = self._bytes_to_sector(self.size)
          sectors = [[] for s in range(sectorsize)]
          sectors[0] = True # locations
          sectors[1] = True # timestamps
          for m in self.metadata.values():
              if not m.is_created():
                  continue
              if ignore_chunk == m:
                  continue
              if m.blocklength and m.blockstart:
                  blockend = m.blockstart + max(m.blocklength, m.requiredblocks())
                  # Ensure 2 <= b < sectorsize, as well as m.blockstart <= b < blockend
                  for b in range(max(m.blockstart, 2), min(blockend, sectorsize)):
                      sectors[b].append(m)
          return sectors
docs: Return a list of all sectors, each sector is a list of chunks occupying the block.